The Finisher...

well, something i do quite often as vasari as a game ending move, is to fire a single kostrua shot at one of the back enemy worlds, and then send my entire fleet (+a few dozen planet bombing frigs) to thier back world, and spread out the planet bombers so i preety much take out the enemies core worlds before he realizes what hit him.  then, I use my maurader (or build a phase gate on his old homewrold i ofcourse colonized) to jump back out when he comes back to defend.

here is a replay me doing this (diplo, of course) http://www.mediafire.com/?ttzj0nkl5ww

I think it depends on the size of the map more than anything. Usually it is just with a superior fleet (I.e. one with higher level captial ships and a fleet composition to counter his), though building a starbase on his homeworld is always fun.

Usually march straight to their homeworld, put up a starbase, and blast it.  Then proceed to knock of each of their planets one at a time.  Really, in MP if they haven't surrendered by this point it's just bad form, and in SP there's really no challenge left in the AI anyways.

In single player my games tend to turn into turtles. I first scout and identify the 'turtle point' at the map at which there is no passage from one side or the other except through it or through the pirates, rush aggressively to acquire it, typically fight it out with the AI to acquire it, starbase it and completely fortify it, backfill the other colonies, and then develop my economy until it dwarfs the AI's. Once I've attained a massive economic advantage I build up my fleet and then finish the game in a very predictable manner:

First, I wait until the pirate bid comes up and win it. I use scouts to identify which planet they go after as soon as it's determinable.

Second, I pay the pirates 40,000 credits to launch a 1000 supply raid on a different planet than the first.

Third, once I see the massive pirate raid in transit I launch my entire fleet (usually over 1300 supply by this point) on a third planet.

The overwhelming force typically spreads the AI's forces far too thin and takes out three of his border planets. So far no AI has recovered.

I wish the random map generator would generate more maps without turtle points. It'd probably make me a better player to have to win without turtling like this, it's just so effective, at least against hard or lower -- haven't tried my hand at unfair yet. I understand the higher difficulty levels get massive hardcoded economic advantages, though... so perhaps it'll stop being such a good strategy when I move up there. Although, sometimes the turtle point is in a massively disadvantageous position for you and you have to play aggressive or lose because the AI gets 2/3rds of the map. And that's fun and hard to deal with.

Also, having recently upgraded from vanilla to diplomacy I have to say kudos to the devs for upgrading the AI. I was curbstomping the vanilla hard, but the diplomacy hard is making me sweat at spots. 

Hoski spam.

LULZ. hoho spamz. only really works if the enemy fleet is HC dependant... but if it is... it works very, very, very well.

To Khift

Try higher AI levels in diplomacy expansion. And more than 1 AI allied against you. It will force you to devise new strategies.