How do I sell or dismantle or destroy those things once I have built them (or captured an enemy colony with them) ?

As far as I know, there is no direct way to destroy population units, droids included. However, getting rid of regular unwanted population can be effected indirectly. If you transport the unwanted population to one planet, then give that planet to any enemy, the next turn you can reconquer the planet and exterminate the population. This works with regular population, but I don't know if it would also work with droids.

Or you could just transport them to a planet that's already full and and ignore the population limit warnings. I believe androids have the tolerant feat, but even with that, you should be able to kill them off. If you do it this way, you don't have to worry about people being pissed at you for exterminating civilians.

Wow, I played MOO2 for years and never even tried that. It just never occurred to me to send population to a planet that couldn't hold it and expect them to just disappear. Makes sense now that you mention it, though :).

Yea i just tried it and you can just transport them to a full planet. I like buiilding droid manufacturing planets on toxic worlds its awesome!