If someone is leaving you unwanted comments on your comic, the simplest thing to do is delete them. Often if someone sees their comments are being deleted, they'll lose interest and move on. In many cases people who leave aggressive or trolling comments are seeking the author's attention (or looking for a fight), and responding to them just makes it worse because it tells them you are paying attention to them and you consider their comments valid enough to spend time responding to.
If that isn't enough, then the first thing is to ask that person to stop, citing The Ducks core rule: Do not harass or abuse other site members. Whether that person thinks they are harassing you or not isn't important- the important thing is that YOU feel harassed. If it's a specific thing you'd like them to stop doing, then request that. If you just don't want to hear from them at all, then request that. It's nice that admins are there to back someone up if needed, but it tends to work out better if the individuals try to work something out between themselves before calling in the admins. And remember, if you want them to leave you alone, you should leave them alone too.
So, if you've asked someone to stop something and it doesn't work, tell one of the admins (skoolmunkee or ozoneocean).
Include as much information as you can. The admins may be reluctant to act on scant information or incomplete stories, because they try to be fair and don't want to police if they are missing critical info about the situation. It helps if you can provide evidence in the form of links to the comments or posts in question, copy/pasting from PQs, a history of that person's behavior towards you, etc.
Please keep in mind they can only act in response to what occurs on The Duck. Any activity on other websites, in private e-mail etc. are something they have no authority over and can't be used as part of a harassment complaint.