What you do, is tell them with a deadpan face, that you are a ninja and part of a clan of ninjas and if you told him anymore you'd have to kill him.
but be serious about it. I say out creep the creepers. Scowl the whole time. Never crack a smile.
You could also go the demonic/I worship satan route. Tell them you're a satanist, that you've received a demon inside you, and you let him loose when you feel threatened. Then describe how he particularly likes the artery in the neck because it makes such a nice blood spray. Again, be all deadpan, never crack a smile
Tell him your coven is looking for sacrifices if he's interested (then have a fake address on the off chance he buys it...)
Another good one, that I've seen my aunt use (she's just like you attracts a lot of creepers) is to pretend to be batshit insane. She starts talking to someone that isn't there and includes them in the conversation. It even creeped me out a bit and I knew she was faking.
Or just tell them your boyfriend is a navy seal and he's very possessive. Look around all worried like, like he might be right behind him. And say you shouldn't be seen near each other.
There are so many fun choices
But whatever you decide to do, just practice scowling in the mirror. Practice the deadpan "Prison face".