Many amphibians are killed by small amounts of caffeine especially frogs. Caffeine is not that expensive if bought from a chemical house.
Another idea is that the toxin from cane toads can sell for several hundred dollars a gram and is used in traditional medicine in China. Turn your pest into an asset?
Paint the belly of the toads with chloroform to get them to release the toxin but be careful to wear eye protection as squeezing the parotid gland (behind the eye) can cause the toxin to squirt into the eyes.