Bloody Slacker

If a planet is influenced by your culture, the enemy isn't able to capture it anymore. Just make sure you have a planet with multiple broadcast centers and if there is no enemy culture influence, the enemy will be unable to use the surrounding planets. It takes a while to have cultural influence multiple phase jumps away, but with time, your culture will spread very far.

I find the idea of destroying planets unapealing. You can simply leave a planet and make sure you have spread your culture enough so that the enemy can't capture it. However... Perhaps an end game idea would be destroying stars. For example: If you(r team) pretty much own a galaxy in a huge map with different star systems, you would be given the option to move your fleet to an enemy galaxy and destroy their star. Destoying the star can be done by setting up a dooms