Dealing with a 'Professional'...

The past few days I've had the absolute Joy of dealing with a 'Professional' CG artist in a FB Group, who seems to have issues with watermarks on artwork. To 'prove' his point he took my most recent render, posted, to the group and removed the watermark. He then reposted the altered image in the comments under my original post to (in my eyes) boast about doing it. He doesn't think he did anything wrong and took offense that I called him out about it...
This has since degraded into him throwing thinly veiled insults at me. Yeah... real professional.
I do this for fun. If you like what I do that's fine, if you don't that's fine too... Just leave me be.