Enforcing contracts

I work with a rescue group that has a contract signed by all
adopters. One clause states that if the animal is disposed of for any
reason it *must* come back to our group. We've had several instances
recently of people simply giving away animals they've adopted from us.
In three cases. the animals found their way back to us via third
parties. We would like to take action against the adopters who
violated their contracts. Is such a thing feasible?

Dick Evans