I had this issue as well on my second game.

Like SOSE so far.
Anyway looks like they mention it here: http://sdc1.stardock.com/public/sin/beta/history.htm
Note: If you attempt to colonize a planet that is high in enemy culture, it will revolt. You need to reduce the culture there before you will able to hold it. This is not a bug and is the desired behavior for the time being. We will be reviewing this area carefully over the next few weeks so it isn't fixed in stone and no doubt some values (like the culture decay) will need to have their balance improved.The only thing I might say in response to that is, if I've destroyed the colony and all the boardcast towers then the culture should decay exponentially. Shouldn't be dependent on the culture been overridden by another, plus who wants to switch to another culture if they are just going to be blown up straight away.
Also there should be a modifier for number of ships in system and timer for flip which affects the colony revolt system. And if you managed to destory the colony and broadcast towers, then the culture should retreat quickly.