One of the best ways to show your largess to other factions to keep them happy and not clammoring to steal your riches is to give them a little stipend of regular tribute. I find that amounts lower then 5 years don't seem to effect it, or amounts less then 300-500 a turn.

Are assassins the ones that stop assassinations? Spies merely detect other spies within a city?

Also, you have to be careful in what you use your agents for, I just learned of the agent limit in my game as epiros. I could only have about 7 spies, one assassin and one diplomat.

What's the use of diplomats really other then carrying tribute to neighbors? You can sit them in a settlement as an ambassador, but what does that really do for you? Is it perhaps for a big empires use later on when corruption and loyalty problems are of concern for other diplomats to be able to bribe your people?