Wow! I would have been pissed too. Don’t feel at all guilty about deleting that author. You certainly showed more patience than most editors would. If he can’t even understand a possessive apostrophe he probably shouldn’t be writing, or needs to do some serious study. Hey, I make plenty of mistakes with placement of commas and stuff like that but if I make an error I appreciate being told about it so I can correct it and not look like a second rate writer.
You have only ever been polite and helpful in your private notes and suggested edits, Martin. I know it doesn’t feel good having to delete anyone, but in this case I’d say, “Good riddance.”