[Suicide Junkie]

1) Either
- Colonize all the worlds before they do
-OR-
- Have a treaty of "None", "WAR", "nonagression", or "non interference", and station ships at all the warppoints with orders to destroy any colony ships coming through.

If you have boarding parties, you can take the colony ships for research or to colonize more worlds cheaply

2) Yep. Try not to forget about 'em!

3) At the beginning of your turn, you should see the ships traveling through. Otherwise, you can watch the "colonies" screen to see the facilities & production going up.

4) 100 turns, yes. I was playing something similar, acting as a neutral (never leave home system) and I only managed to get to turn 300 before I was forced to attack the second-place AI since my star was about to nova! They are very firmly entrenched around my homesystem.