This is a little trick I came up with after one of my capital ships got trapped when several neutral AIs randomly decided to permanently park their ships next to mine. The concept is extremely simple: station a ring of non-military ships around a shipyard, and then the ships inside can't get out unless the owner declares war and breaks the blockade. Unsurprisingly, the AI doesn't know to build a new shipyard in order to counter this ploy. If you block all the race's shipyards, you can build up a force to take out all their worlds in one turn, knowing exactly what defenses you have to beat. If you set up the blockade soon enough, you can prevent them from colonizing more than a couple of worlds, although it's probably not a smart play with more than one opponent, since you'd have to stunt your own growth quite a bit in the process.
Here's a picture from my proof-of-concept game: http://i.imgur.com/HBqmp75.jpg. Just one godlike opponent on a tiny map. You can see nine ships stuck on top of the shipyard. I rushed Planetary Invasion, and with all the AI's combat ships stuck on the shipyard, I just waltzed into each of their 3-4 worlds with an unescorted transport. I have this idea that I could use this exploit to pull off a single-world godlike conquest victory on a medium or large map with several opponents (incidentally, the test game itself was a successful single-world challenge, although that's not very impressive on a tiny map).
The one thing that can foil this tactic is if the AI randomly decides to leave a ship in a space both next the shipyard and a planet. That ship will create a bridge to the planet which you can't block, allowing it to move defenses to the planet and eliminate the guarantee of a one-turn victory. Your best bet if this happens would be to buy that ship, or possibly to bribe the AI to attack someone, in the hopes that it decides to use it for combat.