Good player, first of all, but I can understand your fustration. I had the same problem for awhile, since we did the rule on neck breaking.
Anyway, he seems to mainly target mages. Now, this would get around.
"I'm positive, Shim, that's the Fist Brother who busted up Gortun so bad the other month. Ya, him."-Overheard in the market.
Mages are not going to stand for this for too long, after all, they can't have thier reputations being tarnished by some shaved ape who finally learned how to make a fist.
So, the evil fighers line up with "hold the line" and the monk springs off the 1/2Orc barbarian's shoulder, bounces off a shield, and launches himself at the mage, barely coherent thoughts of physical violence filling his tiny little mind.
Herein is your problem. He's rushing the mage. OK, that incurs attacks of opportunity from everyone he just went by.
X-mage
Y Y Y Y Y (Evil Fighters)
M (Monk)
So monk leaps through. He encounters attacks of opportunity from the THREE middle fighters. Middle fighter performs Improved Trip, fighter #2 does Crippling Strike (ala Song and Silence) and fighter #3 takes a 5' backwards step to protect the mage, and performs a knockback attempt on the monk, knocking the sap right back into the fighters.
Solution #2
Evil Cleric, working with Evil Mage, sees the charging monk, who is drooling on himself with the thoughts of laying his hands on another mage, and casts BLADE BARRIER!!! Now the mage has Ironguard up, so the metal just whistles right through him. If the monk chooses to engage, he can eat 20d6 per round. And let him go for the Cleric, who is wearing spiked armor, that has a magical property of oozing venom.
Solution #3
Mage recieves an attack of opportunity.
DIG spell. Look, Monk in a pit! Throw in a pebble and cast Enlarge.
Solution #4
Illusionary mage, the mage covering a portable hole. Monk falls in, mage wraps him up, carries him back to his lab, and makes him into an interesting coffee table.
Solution #5
Mage is a sicko, into REALLY extreme body piercing, and is specialized in touch attack spells. A hug from said masochist will inflict 2d4 points of damage per round, and the mage is a mute, so takes no penalties for not speaking while he casts.
Solution #6
Mage uses his attack of opportunity to throw cayun pepper in the monks face and runs, with his boots of striding and springing!
Another fun thing to try to grapple is:
A bag full of green slime!
Hope that helps.