Actually, that's possible, albeit in a sub-optimal way.
In the GalCiv2/Data/English directory, there are two XML files you need to modify: GC2Ships and GC2Types.
In the GC2Types file, define an additional LifeSupport component by copy/pasting the Basic Support section, renaming it to something else (I used ReducedSupport) and by modifying the range modifier to a negative value (I used -10). In GC2Ships, you'll find the default entry for the colony ship and you'll notice it comes with three Basic Support Modules per default. Swap these with your newly created ReducedSupport modules ... et voila.
However, I have still to playtest this as I figured this out just now, because I'm immensely annoyed by the AI urge to colonize which beats the Civ3 settler flood any day.
I'd presume the AI uses the default CS design, so it SHOULD be affected by this. However, you'd have to retain yourself from designing another CS which outperforms the AI by simply swapping the reduced support modules with basic support modules again.
_____
rezaf