A popular trick among game devs and authors is the "infinite wasteland" trick:
Go anywhere you please, but when you leave the desired zone, you won't find anything interesting.
In space, this looks like the very realistic concept of seemingly going nowhere forever. Obviously, nobody wants to travel for days outside the zone where there is stuff, especially if they have work to do, so having nothing nearby is a good reason to not go nowhere. Why would you leave?
Another trick is the "leave or get shot for defecting" standard:
You run off, we will point the big guns your way and blow you to Saturn.
This very nicely limits your "actors" to a bubble, or multiple bubbles, which they must not leave, lest they are destroyed. Play any FPS and you will understand. Your commanding officer does not approve of them leaving.
Or you could use the "you drift into space, helpless" trick:
You run out of fuel, and now you're hosed. You float into eternity.
Some resource that you need, eg. sunlight, fuel, O2, runs out because the places/bases where you get more are not in your area. People need "stuff," and if they leave their "stuff spots," they will soon die.
Or maybe just the "random instant failure":
You go outside your zone and then some seemingly random event burninates you.
An "actor" of yours decides to leave and takes a tiny asteroid through the face and dies. This isn't necessarily a limit, however, 'random stuff happens' is a surprisingly common trope that limits people in some way, if not obviously.
And finally, "this is our turf":
This is our spot, and since we are advanced, we have 'death-stuff' for keeping you off.
The aliens, who are terraforming as you said, probably have this idea that you should keep away from their stuff, enforced by alien barriers of your own choosing. In the case of "two-dimensional?" aliens, I would suggest a big steel blade that slices you cleanly into oblivion. But again, choose your own adventure.