Oof, no fun.
How do you react, CL?
This was happening at my hubby's dept., the secretary was being bothered by a guy and it was driving her bananas. He (hubby) advised her to just say something flat-out; I forget how it went, something like, "I'm a professional and demand to be treated as such," said firmly and with eye contact. The guy was abashed, and stopped.
If something like that doesn't work with this guy, you have a case for harrassment.