What would you do if someone seemed to be trying to initiate an emotional affair?

I recently reunited with a high school friend on Facebook. Now, it could be my imagination, but I think he is trying to start up an emotional affair. He told me the other day that he wanted to date me back in high school (25 years ago) but never found the nerve to ask me out. Whenever he sees I am on he pings me and he sends me notes that end with x’s and o’s. He has also shared stories with me of what seem to be emotional cyber affairs he has had in the past and shared the fact that he does not feel passionately towards his wife.

I have been very clear with him that I am in a dedicated monogamous relationship. Should I cut him off? Just hide when I’m online? Or might I just be seeing things that aren’t there?