There's a lot you can do to make tackling a mage a unique experience every time. If your party has developed a single tactic for dealing with mages, that just indicates your mages come from the same mold.
Break that mold. Design a mage specifically for grappling. If you focus that mage on grappling, he'll handle it quite well - especially with a little warning to buff up, he can probably out grapple anyone in the party ('cept maybe at lower levels)
Focus the magi in your games away from what you've used as a mold. Pick specific concepts like grappler, tripper, melee expert, ranged expert, etc. You'll start finding concepts that neatly defeat the 'charge' en masse tactic.
From the sounds of it, you have more than a few players - ever consider tossing in additional mages? Extra help from unexpected directions...
For that matter simulacrum is a high level spell, but a simply wax sculpture of a mage can ofter decoy quite successfully - especially when combined with any of the phantasm, illusion spells.
Heh - make 20 such statues, use mirror image and illusions. Slice and dice the party into ribbons as they go charging about trying to pick a target
Best chances of avoiding a grapple in order: no be there, gain super bonuses, and inflict super penalties to them like mad. Size bonuses, oiled skin, multiple arms, summoned monsters, etc.
Be wary of magic items on the wizard(s) and guard(s) - your party will get those. Use one time items (lots cheaper to make anyway), and hand out a few to the guards... a potion of grease to each of 10 guards - someone is going to beat the players, and grease is a wonderful obstacle to charging foes. Tentacle spells, etc. also help. An illusion of a solid floor covering your bath pool filled with acid is just plain mean to place between you and those charging grapplers. Provide all of your guards with one-time use, 10 minute levitate devices to stand on the illusion with... Presto, no more grappling.
Just change your mage mold. You'll shock those players into incoherency.