You can always play with "Players can only colonize home planet type" enabled.
That makes it impossible. Personally, I think that colonization tech is specific to the race and should NOT be either capturable or tradable. You should have to develop the environment controls for your own race. But then I also think that you should have to develop a bio-weapon specific to a given race. 'Generic' bio-weapons don't make any sense...
[This message has been edited by Baron Munchausen (edited 01 November 2001).]