Oh no. Please don't use a BB Gun on them.
I would put the patience into trapping them humanely.
Besides, Feline Leukemia is an airborne disease so if they've been in close contact with your cats already, there's no way to prevent it. I used to work at an animal shelter and I've lost 2 cats myself to leukemia. I had one cat and then adopted a feral cat and it turns out he tested positive any by the time we got his test results back, my other cat contracted it and they both had to be put down when they got really ill. 