Whenever I meet someone, I immediately alert them to the fact that if they screw with me or my friends, I will obliterate them. So far, it's worked out pretty well and I haven't had to obliterate anyone yet. Do you know why? Because I let people know beforehand what would happen if they screwed with me. That's why.
There's no better way to start a relationship than with a direct threat. Sure, they may know the threat exists. But it works better if you make it explicit. Trust me, I've been doing this for years. No one screws with me or my friends. Period.