how do you discourage tag-along boyfriend occupants

[Dan C.]

How do you discourage tag-along boyfriend occupants?

I almost rented a place out to a young woman but it ultimately did not work out.

However as it fell through I recognized that she had fully intended for a boyfriend to move in with her.  I had met the boyfriend once, but he had never applied and it was not mentioned initially that he would be living there.

I write on my qualifications that everyone 18 and over must fill out an application.  Any other thoughts on ways to discourage this.