I haven't seen anything really posted on this yet. so here it is.
I have played several games now against the AI with friends and after taking removing all Alien influence from a world by destroying his structures and the planet pop. the Culture is not detracting from the world. its staying constant. This even happens when you have a fleet of Cap-ships in the gravity well which is supposed to remove the Alien influance, this also causes you to not be able to take over the world.
I have also noticed this on my planets when I have a culture building built in the grav-well and the alien culture is piercing through my world to another. Even after building the media center the influence will not detract. I have however noticed that once I build more then one media center in the well it does after a period of time start removing their influance.
I still have Save games if needed.