J.B.
I believe you can tell her that he won't be allowed to be an overnight guest, but at 26 I don't think you can tell her who to become involved with regardless of the fact you guys are paying the bills. You can speak to her about actions that may come out of this relationship and if you aren't happy with them, you can ask her to move, but at 26 who she dates is really up to her I feel. I have some friends in a similar position and they have told their daughter that if she wants to be with so and so, she can't move in with them, only bc she starts lying and doing things that aren't good for her or her relationship with her parents, but they have never told her she can't date the man as she is a grown woman. Good luck!!