Make friends with asexual men or don't make friends with men at all. Even if you lay the cards out on the table when you make new male friends, there really isn't anything you can do to stop them from 'falling' for you aside from acting like someone you're not. It isn't too hard to realize it's something humans have much control over.

Unfortunately we asexuals only make up 1% of the human population, so good luck with that.