I had a southpaw in my club who always landed right hooks. So what i did once is stepping on the left to invite him to take his shot, he launched a right hook (i was hoping for it) just had the time to duck under and give hime a right hook to the body. But i didn't try it afterwards and circled to the right with a southpwa stance too, so that i could keep him away with strong jabs with my right. One other thing to try would be to catch his right bicep with your left and then land a right, but i guess it requires a lot of skill, never tried it