Sounds like you have evil alignment. One way to solve this is to have a (far) higher military rating than the AIs, that will keep them from trying to extort you. There may still be occasions when they declare war on you but this will happen less frequently.

The Thalans seem to be doing best in small to medium galaxies (thanks to their initial high output buildings) whereas it's the Torians in large / huge / gigantic / immense galaxies thanks to their population growth.
Personally I prefer the Korath.

Which civilization are you playing?

Hmm...

The thing is that the Thalans are highly belligerent, If they have a bigger military than you, then they will attack you (after extorting you, maybe).

Same thing with the Arceans. Both of them tend to be heavy on the military.

Here is a hint when fighting the Thalans:

Invest heavily in Shield Technology. The Thalans seem to never use anything else but beam weapons so getting thick enough shields will help against them.

Beware though, they have the powerful technology known as the Disruptor Beam Weapon. Once they are out, then prepare for some serious hurt and noise (seriously, the sound effects for them are too loud!)