Hypersmurf
Moderatarrrrh...
Does someone know if you can stop a AoO from taking place?
An AoO is an option, not an obligation.
Otherwise you'd be required to punch your cleric when he tried to heal you - he's casting a spell in your threatened zone!
Anyone is more than welcome to forego an opportunity if they expect a better one might be coming along.
As a DM, I always phrase it to my players "You can make an AoO, if you wish"... usually, of course, they take me up on the offer, but occasionally they choose not to.
The occasion I remember was an assault on a kobold lair. When they entered the guardhouse at the entrance, there were a few kobolds inside - a couple with bows, one standing near a gong.
All three were in the threatened zone of the first fighter through the door.
One of them went to shoot the fighter with his bow. I offered an AoO - the fighter declined.
When another one went to ring the gong... then he took his AoO.
I suppose you could come up with a custom spell or magic item - like Nethack's "Ring of Aggravate Monster", maybe - that forced people to attack you at any opportunity... it'd be good for your super-high AC Mobility damage sponge, maybe, so he can tie up all the opponents' attacks while everyone else stood back with spells or ranged weapons...
... but otherwise, AoOs are always optional.
-Hyp.