[Baalz]

Well, personally I consider several things hostile actions which I'll take as a breach of NAP and feel justified in immediate hostile action. To me a NAP is just an agreement that we shouldn't waste our guns pointing them at each other because we can each be stronger by focusing elsewhere. If you're going to (in my mind) try to screw me over and try to hide behind the NAP you'll find it precious little cover. Don't get me wrong, I'm willing to overlook a little bit of friction, but I generally consider it an act of war (as far as I can detect it) to systematically:

Corner me in by conquering all indies in my path of expansion.
Build several temples on our border
Take provinces I lost to a random event or to another player
Cast anonymous offensive rituals at me
Raise unrest with spies
Try to assassinate/seduce my commanders
Move stealth troops into my territories
etc.
etc.

In short, I feel no obligation to honor a NAP with someone actively working against me.