Oh starryprince, I am so sorry this happened to you. It's not OK for anyone to do that. I like Jan1212's suggestion. I'd also suggest to document what happened and the any subsequent interactions you have (including any discussion about boundaries and professionalism). You might also want to check the organization's website to see if they have any policies on workplace harassment. There might be some tips there that you could use. If you are uncomfortable approaching this person, it's really OK to go to the HR department. That's what they're there for.