1.) Keep herons and other fish eating birds out of the pond via staked lines. Many fish eating birds wade into the water and if a couple of lines at the correct height are installed the birds can't wade in.
2.) Make the banks steep enough to discourage wading by fish eating birds like the heron.
3.) Plant the pond with redear sunfish to consume snails that are part of the cycle which produce these.
4.) Treat for snails along the shoreline with copper sulfate.
5.) Reduce aquatic vegetation that that snails feed on.
6.) Mow vegetation on the shoreline that snails attach to.
Here's a good read:
http://www.luresext.edu/aquaculture/yellowandblackgrubsinfish.htm 
