You can clean it with glyoxide. It is an oral antiseptic. It isn't cheap but it will make sure it doesn't get infected. Your mouth is one of the dirtiest places on your body. You can tell her something silly like when mom used to say, "stop making that face, it's gonna stay that way!" you could make something up. (you will grow a tooth there if you keep touching it, LOL) Or you could tell her if she keeps touching it she is going to get hurt and have to go to the doctor.
It may be that the speach therapy has brought attention to the roof of her mouth creating the facination. You could teach her that it is ok to do it with her tongue. Teach her how to roll an "r" like in spanish. That will still give her the tickling sensation but with a soft touch. Good luck!