On thing to note, as soon as the weight of all the grabbled characters exceeds the wizards max load, he or she can only stagger around (5'/round) Any more than double max load, he/she cannot move at all.
As for how much weight is necessary to pull the character off the ceiling, take the necessary strength to pull the caster off the ceiling. As soon as the load exceeds double the max load of one less than that strength, the character comes off the wall. For example, in your case, a strength of 25 would pull the character off the ceiling. With a 24 strength, someone could stagger around with up to 1400lbs. As soon as you hit 1401lbs, that would require a 25 Str, and the character would come off the ceiling.
You could rule that any weight more than double the caster's max load would inflict subdual or even regular damage each round. If you have a 20th level caster with a 10 Str, it is clear that hanging a 12800lb weight from that character's waist would simply tear him/her in half instantly. Gotta love D&D math. But in that case, a 20th level party could use the wizard as the anchor point for a pendulum to swing a 6-ton statue across a chasm or something else of equal ridiculousness.
-Fletch!