Ill just throw out some ideas on ways to combat the whole MobSpawning area thing.
1) Do a serverwide Delete of all mob spawners. We have a contained area (10,000 x 10,000 I believe) so do the whole thing.
2) Though it may be severe, limit Creeper, Zombie, Skeleton, and Endermen to the Nether. You want the nether to be THE place to go to get money, if the mobs are all spawning there, and in bulk, it will be.
3) Remove the rule (I don't like this one, but on the other hand, I can't imagine that farming mobs like that would be any more productive then going to the nether)
4) MASS Law enforcement. Mods watching everyone, all the time. Cameras in the woods, watching everyone's every moment.