Ok, I read of lot of sci-fi. I'm pretty sure that unless we get invaded by yellow blooded cannabalistic omnivorous alligators (ty John Ringo) we have little to worry about.

Our food will more than likely not be compatible with their food. Our atmosphere may kill them. Our germs either will zap their butts dead, or not effect them - 50/50 chance. Unless they are way advanced our weapons will hurt about anything.

"Scuse me Mr. Alien, stand here while my buddy shoots your green self with his Barret" "Wow, that's some hole. Does it hurt?"

Just load up on ammor. Maybe the pets can eat them.

Failing that, use the towel trick from Hitchhikers. If you can't see them...