I haven't found it yet.
I'd like to bribe or extort my enemies into not attacking me constantly. I've built up outposts, defense stations and added defense to each of my important systems, but the AI still declares war and attacks, constantly. Is there ANY way to prevent this? Am I being too aggressive? Not aggressive enough?

There should be some way to deal with the AI other than completely destroying them.

Play teams! So eg in 4X player have one on your team and every one else solo. A dumb-bell galaxy (mirror image) should put you on one edge and your team mate on the other, then you build towards each other.

http://pukdarkstar.businesscatalyst.com/images/teams-crop-u21306.jpg

Send some money/ ships to the team mate now and then to help him out and the game will run nicely.

Also try demanding for their subjugation. Surrender in SR2 means that you become a vassal empire of the Empire you surrender to - this allows for you to effectively defeat, and then add to your own power, the Empires of the galaxy.

If the AI believes they can attack you, or feels they need to attack you (because they need to expand, their resource systems are being compromised by another Empire, etc.) then they will.

Contrarily, if the AI believes you are much stronger than they are they'll be more open to surrendering to your Empire.
If you don't mind teaming up with the Empire that's interested enough in you to attack you constantly, surrendering to them to become a mightier union is also an option as from that point forward you are allies of equal footing whether you like each other or not.