couple of ways...spam broadcast towers at an adjoing planet...if advent, use deliverance engine... or fly fleet to next planet and blow up the oodles of broadcast towers there then flyback adn wait to colonize.

the advent super cannon (the eqivilant to the novilith) can culture a planet to death.  if its a teammate, blow away the enimy culture so you can have a little time to settle it and get some broadcast structures up.

Basically, having capital ships in orbit will repel enemy culture.

The culture will dissipate more quickly if you have broadcast centres in the adjacent system(s), or if enemy broadcast centres are removed etc, but either way, leaving your capital ships in the grav well will eventually allow the planet to be colonised.

It's not very realistic (why would an annihilated and unpopulated planet care about culture?), but it's a nice way of slowing down an advance into an enemy stronghold.

What classifies enemy?

Sure it is.

Instead of thinking "unpopulated" think "taxation and other governmental infrastructures annihilated". Those population points aren't really people (although the population are figured into things in terms of potential), it's more a measure of how much you can tax the world, and how productive it is. Instead of thinking "Culture" think "The will to endure as a people and put up strong resistance to enemy subjugation techniques".

Day to day governmental functions tend to be poorly executed when the land has been reduced to rubble. When the culture drops to zero and the world flips, it's more akin to surrendering... or having been devastated to the point where the local government no longer functions, and proper resistance is no longer feasible. At this point, one's ground forces/colonists/administrators can conquer/settle/dominate the world's indigenous population, and begin to rebuild.

-Itharus