Going along with what MemsHearts was saying.... Throw salt at his feet without him noticing.
There are plenty of spells in this website's spellbook. Warding is another term for protecting. It sounds as if you may mean banishing or binding. Spells are not going to force someone to go away automatically. It is more of a gradual thing that happens when it comes down to physical beings.
It is also important to remember that spells can come with a price and whatever you do is being done to yourself too. Think about the possibilities before performing any kind of spell. There are probabilities that you may harm your girlfriend even in the slightest way. What are her thoughts about this situation? Could you be over-reacting about it or are you jumping to assumptions based on his actions? You should talk to your girlfriend. If she is loyal and is committed to the relationship then she will most likely not go after another person.
No one can stop you from performing the spell. If you think it is the right action to take then feel free to do so. Follow your intuition with it.
I suggest not using magical because it comes with a price and it's never a good idea to use magic to harm because it will some back to bite you big time. I simply suggest a verbal confrontation and if that doesn't work, then a physical display that you mean business is in order. I do not condone fighting but i understand that sometimes it is necessary.
Hope this helps.
While spells can help, try also talking with them. Tell him to leave, or bring up your feelings with your girlfriend, or both. If the question is less flirtatious and more aggressive, and your girlfriend is scared, might want to get the law involved if he doesn't leave you two alone.