There are only so many snails an Assissin Snail can eat in a week (3-5 I think is typical) and if you are overfeeding the tank so much that the unwanted snails are breeding like rabbits (but not Rabbit Snails), then your population of unwanted snails will keep increasing.
Spend at least 5 minutes a day catching unwanted snails yourself and you ought to eventually sort this infestation out. I had a real problem in my 620T last summer and I eventually eliminated Pond Snails from it by daily manual removal foir 2-3 weeks, the amount of baby snails I removed was staggering (in the hundreds) and the tank is now completely free of them.