There is something important to remember about humans.
We are not at the top of the food chain because we are terribly durable or strong or agile. We don't have natural camouflage or natural weapons or anything else like that.
Humans are at the top of the food chain because we are smart.
Humans are really good at deforestation. I mean...we're kind of too good at it. And you're dealing with space age humans. Herbicides, fire, heavy logging equipment, bringing your spaceship into a low hover over the trees and flattening them with rocket-wash, and so on.
If you want a simple solution, start your colonization near the ocean. Assuming this planet's geology is anything like Earth's, then land near the ocean is saturated with a high water table. This makes the ground less 'sturdy' meaning massive plants won't be able to stay upright long enough to become massive. The ground just isn't solid enough to handle their size.
So, by starting near the coast, we're starting with shorter plants. From here, we move on to standard Slash and Burn tactics. Cut down the trees and burn them. Whether you cut them down with axes, saws, lasers, rocketwash, sonic cannons, or whatever futuristic method of destruction your space-age colonists have--doesn't matter. Knock stuff down, set it on fire. This purges the area of plantlife, insect life, and is a good first step towards the ecological reset you need to perform in order to get Earth crops to grow in alien soil. Once you have your foothold, you can spread inland, carving a fiery path through the foliage.
Or, heck...there are no settlements down there that you care about. Start some forest fires. Sit back, watch em burn. Toast Marshmallows. Humanity has a long, proud tradition of clearing land with fire. No reason to stop just because we moved to a new planet.
Humans are really good at dealing with natural threats. We basically have two responses to things that are able to hurt us.
- Kill them first. When a tiger attacks a village, humanity doesn't huddle in their homes and be thankful they weren't taken. They get/make weapons and go kill the tiger. If the weapons don't work as well as we'd hope, we innovate until we make ones that work better. Humanity has historically driven species of predator to extinction because they messed with us.
- Make Friends with them. Once upon a time, there was a pack hunting animal that was a tremendous threat to humanity. We abducted their young, trained them into hunting partners, then (many, many years later) had sufficiently screwed with them via breeding that we made Poodles and chihuahuas.
So, for Option 1. Kill mature Eco Titans in the area. As they have 'muscles' that consistently contract, they are going to operate at a higher temperature than surrounding plantlife. Finding them won't be all that hard. As you mentioned in your prior question--it takes them 500 years to reach maturity. In the most recent 500 year span, humanity went from "Wow, it is possible to sail all the way around the world" to "We are making plans to go live on another planet soon" and from "Muskets sure are nifty!" to "We can sink entire islands with a single bomb."
Sure, killing an adult Eco Titan makes more Eco Titans. But it makes baby Eco Titans that won't be full-grown for 5 centuries. Sounds like a fair trade to me. And hey, in that time, I'm sure humanity can figure out how to detect and destroy their spores. As mentioned, we're really good at killing stuff.
A few options that already exist...
A modern Thermobaric Bomb detonates at a flash up to 5,400 degrees Fahrenheit, a full blown Forest Fire peaks at merely 1,472 degrees Fahrenheit. The blast of a Thermobaric weapon is enough to instantly reduce any plant matter caught in the immediate fireball to ash. And, of course, set fire to just about anything nearby.
Orbital kinetic bombardment (as well as nuclear and thermonuclear weapons) can release enough energy to reduce all matter caught in the immediate strike to plasma, by emitting sufficient energy to shred the atomic bonds of the molecules, ripping off all the electrons and flinging the atomic nuclei apart. They'll largely put themselves back together eventually, but certainly not in the same configuration they started in. I don't care how sturdy your plant's spores are, being shredded at the atomic level destroys anything.
Chemical Warfare can let you destroy these things in myriad ways, possibly including ones that don't trigger a release of spores. You kill an Eco Titan normally, then contain one of the immature ones that results from it. Then you experiment on the sucker, taking samples from its materials and so on, until you find a chemical cocktail that harms them. Then you just deploy that as a weapon, and continue to refine it while you go.
Option 2 is the 'make friends' or at least 'make it useful' option.
If you know where an Eco Titan is (they're immobile) then they can be useful. Depending on how exactly they catch their prey, you can avoid that--either by keeping your distance, not falling in the hole, whatever. Put up warning signs, caution tape, so on. If it emits some scent that lures people in--heyo hazmat suits and 'minimum safe distance' markers. If it has a 'hole' that prey would normally fall into, then seal it off. It's a plant--it can't reach up there and pull off the steel platform you just mounted to its orfice.
Now, how can we make it useful?
- Bio-waste recycling. Eco Titans eat biological materials...dump organic trash into their mouths. They get fed, we get rid of some trash.
- Useful materials. If (as was suggested in the answers to your other question) they produce something desirable as 'bait,' then once an Eco Titan is made safe, we can harvest that something. If you can't physically approach it for some reason--well...humanity did invent drones for this sort of thing.
- Study. These things are huge and sound fascinating. Let the egg-heads poke and prod. If nothing else, they can figure out what makes these things tick, so we can figure out how to kill them better the next time we need to clear space for a village.