Or tell him to polish your caber.
I did some looking around, and apparently you can get in trouble for "internet harrassment" if you direct a message towards someone with the intention of inflicting emotional harm. I do not think that posting a comic here exactly constitutes direct harrassment, though. (Then again, there is probably a lawyer out there who could figure out a way to define what you are doing that way.)
As long as you do not continue to antagonize this person, I would think his or her case would be rather weak. Harrassment usually implies repeated actions. Without a pattern of harrassment, most cases can be simply dismissmed as a single case of bad judgment.
But don't quote me on this. Free legal advice is worth what you pay for it...
---
"And Wirthling isn't worth the paper he isn't printed on."