I have yet to see any of the AI's dismantle a building on any planet that they take over. Which made me start to think...
Colonize a planet build a quick starport build another colony ship move on..
Dismantle starport put in whatever else you want researh, capitals unique items etc.. This way when an enemy captures a planet they don't have the ability to protect it with ships. When you capture an enemy planet remove their starports. Instead build up your planets with planetary defenses. Picture a PQ 32 planet with nothing but Defensive buildings maxed out.
Eventually you would cripple the enemy since they have nothing building fleets. All the while your researching the nasty nasty damn damn techs and are making fast fast heavily armed and protected ships. that do nothing more than take out what little fleets they have left. Over the course of simply time itself you would whittle them down to only 1 planet that is producing a ship a turn. Even if they have a couple of rogue transports that capture a planet or two they never seem to build starports. They only upgrade the current buildings.
The nice thing about this idea is that they have to create fleets further away from your worlds and they tend to keep their fleets moving around. letting their new worlds produce more fleets but with no starport theres no fleet production. I can see this making the AI really upset.
After all this is a game of attrition when you boil it down. Simply deny them the ability to produce and fleet up..