Favorite methods to defend casters from grappling monks:
Ring of Freedom of Movement, you auto succeed at all attempts to shrug off or escape grapples.
Mestil's Acid Sheath, you damage people in contact with you.
Anti-Life Field, Repulsion, or Fly, you play keep away.
Mislead, Illusions, Invisibility, you give him someone else or nothing to grab.
Summon Monster, you summon a bigger monster to pin him. Size Bonus + High Strength means many monsters will out grapple a monk. Dire Bears are a personal favorite. Upper tier elementals are also very good.
A Rogue Bodyguard, a grappled monk loses his Dex to AC, on top of making him easier to hit, he is now susceptible to sneak attacks. A dual wielding halfling with daggers can do upwards of 25d6 damage, no save

to a grappled monk.
A second spellcaster. Which one will you grapple? Both of us are deadly.
etc. etc. etc.