Yes, because going up to someone on a forum that has nothing to do with the forum in question and telling them that they sucked at being an administrator is something that needs to be told to someone. You don't just go up to someone and start talking like that. Don't bother arguing about why you did it. The bottom line, despite the fact that it wasn't written, he asked you to stop, you didn't. You continued to bother him about the subject regardless, thus harassing him.
Regardless, this isn't up for debate. Just in the future, don't bother with things like this.