In my experience if a terrier is getting enough foxes, he won't lie with a rabbit. However I can recall a couple of good game dogs lying with rabbits in the past. If we're talking about youngsters, just discourage them, they'll come round. No I wouldn't be pleased about it but I certainly wouldn't put them to sleep. I must add, I've never heard of anyone adding bull to make a terrier a bolter!!!
Stevie