Harassment is not okay, period.

I also feel that responding with “I have a girlfriend/boyfriend/partner..” is not the best statment to use.
Using this as a response tells the harasser that his or her behaviour would be acceptible if you were single. It is a response that may seem easy to use, but more direct responses need to be said. “You are making me feel uncomfortable.” and “I’m telling you to stop.” cannot ever be misinterpreted.

I would suggest collecting evidence of the incidents and telling a boss/human resources to start with.