There's no rules against it I believe, fortunately you should be able to delete it. Don't respond, because they usually get defensive and go full troll on you, just delete it. That's how I usually deal with them. Unless it's not really self advertisement and it's a reader linking another comic they read because it relates to a comment or a page, I honestly don't have issues with ALL advertisement, just intrusive advertisement that has nothing to do with your comic.
I think one of the worst (none bot) spam I encountered was someone that would post on my wall "Thanks for the Sub!!!" and then have a picture and link to their comic... which is, sure, okay... BUT. I never subbed to their comic. At all. I've never even seen it in my life. They were posting fake 'thx' comments on peoples walls to try and hide the fact they advertising their comic on your wall. It was fuked up.