You can try a product called no planeria from someone, it's got the usefull side affect of killing snails, not shrimp though. You'll never kill them all though, I crush them when I see them in my shrimp tanks, I underfeed too, they graze the biofilm and the scraps from the fish. The only thing you really can do is control them now, the no planeria will kill a heap but some will survive. Snail killers will also kill shrimp, their main component is copper ions. No planeria is made from some nut which won't affect shrimp though.