NASA received a telegram from the direction of Proxima Centauri stating that the abominable snowman are coming for a visit and they expect snow to fall everywhere even along the equator. They also promised all out war if we didn't give them a snowball Earth and ETA is less than a century away! BTW according to a few of the defectors still hiding in the Himalayan region claims that the abominable snowman once drained the entire ocean from Mars and polluted the entire sky on Venus. (However they seems to put the blame on Bigfoot, another alien species for tilting Uranus.) How shall we plan to throw a year round x'mas party to welcome the yeti so as to protect the last hospitable place in the solar system? To ignore the warning is courting death!
Well, I think your scenario is an unsovable one by conventional means -- either humans do nothing and are eradicated by the Abominable Snowmen, or they actually manage to cool earth down enough for snow on the equator and pretty bring their own doom about with that.
Obey and kill yourself
Cooling earth down enough that there is snow even on the equator means somehow reducing the general temperature on the surface by a considerable amount -- I'd estimate 6 degrees Celsius or more. You can manage something like that by having multiple huge vulcano eruptions, gigantic meteor impacts, huge nuclear explosions (detonate the world's nuclear weapons all at once?) -- pretty much anything that has enough energy to blow a huge amount of particles straight into the stratosphere. They reflect the incoming sunlight so that it can't warm earth, and if you make the layer thick enough I bet it would only take a couple of years for temperatures to drop that much.
The Year Without Summer shows that this strategy works pretty well. On the other hand, it also proves that even a 'mild' volcanic winter for a single year is pretty hazardous to living conditions. If you scale things up to the degree you need for earth to cool down that much, I am pretty sure you're bordering on an extinction level scenario there. The dinosaur extinction event apparently has places where earth / the sea cooled the amount of temperature you are talking about to get earth to become a snowball:
At Brazos section, the paleo-sea surface temperature dropped as much as 7℃ for decades after the impact (Wikipedia)
The result of that was that no land-going animal species above 25kg survived. Not sure where humans would fall in this scenario, but if the human species survives there will be at most a couple of million of them left. And they have bigger problems than trying to throw a welcome reception for the Abominable Snowmen.
Realistic human reaction
Realistically, humans will completely ignore the danger because
- the telegram is a fake (how could the ABS learn human language well enough to communicate???)
- the few Yeti are crackpot apocalypse believers -- the number of 'apocalypses' that have been predicted for earth are staggering, and nothing's happened during all that time
- if I spend money on trying to develop an anti-ABS plan, I will get heavy criticism for not spending the money on current human problems, and the next election is in two years. I want to be reelected! And anyway, in a hundred years I will be dead, so what should I care?
- technology is progressing fast enough that surely we will have come far enough in fifty years to find a much better solution in a fraction of the time it would take now, so why should we make an effort now?
- this is a world-wide threat, so all countries should help along. After all, a scenario like that already works so well for the world climate conference...
It is practically beyond our reach, but you could try these methods.
Begin by removing all sources of methane on the planet. Start with cows and gradually exterminate all mammalian species one by one. Eradicate most of the humans too, in the final phases, only keeping a population of not more than 10 million on the whole planet.
Shutdown all power plants at once! Oil and gas drilling must be stopped immediately! Coal mines must be closed instantly!
Plant trees everywhere to fix away carbon dioxide from the atmosphere. Make cutting trees a capital crime.
Shutdown all industries and ban all automobiles.
Try and constantly spray the outer layer of the atmosphere with very fine dust and soot particles for a long time (Each country must do it daily for 50 years). These particles will block sunlight, gradually decreasing the temperature on the planet. Notice that only the very high altitude planes would be the only vehicles authorised to burn fossil fuels.
With some luck and prayer, your man-made snowball Earth scenario will set in within 80 years or so.