There are only two non-toxic ways to control Bullfrogs. One is to kill them directly, one by one: time-consuming, labor-intensive, and less than 100% effective but no adverse side effects. The other is to drain the pond before that year's tadpole cohort has metamorphosed. This can be done in the fall when native tadpoles have metamorphosed already; Bullfrogs take two years to complete the tadpole stage. Of course, you'll kill the fish, too. There's the added problem that the source population will still exist. Small-scale, local control efforts are rarely successful in the long term and require constant repetition. You cannot maintain a Bullfrog-free pond with other organisms in it if there are other ponds nearby with Bullfrogs in them.