Thanks for all the ideas... the next encounter should be, interesting. I don't fault the player for using his tactics. If roles were reversed I wish I could come up with something like this. However, I have a very special encounter planned for him, and I don't want it to fall apart because of this oft repeated tactic.
[edit: Oops. ]
I think the root cause of my problem is the tumble skill. With a tumble check of 15, you can tumble 20 feet w/o drawing an attack of opportunity. Given a tumble of 14+, this means they can tumble past an enemy line as their move w/o difficulty ( a 1 on a skill check would only give them a 5% chance of failure at this maneuver ).
So, once past the enemy line ( or horde, or whatever ), he gets his attack on the enemy caster. The caster can't be right behind the line because one of the PCs has a weapon with reach. So then its the monk and the caster with a low attack, who isn't gonna land a hit most of the time... and the caster is out.
Definately 2 rogue body guards will be added to the next encounter... possibly with rings/potions of invis.

An illusion or other trick in there as well.
Hmm, maybe I should make a feat or skill that allows AoO against tumbling characters... or at leasts raises the incredibly easy 15DC.
Or maybe I missed some errata somewhere...