If it's clear who the user is because they've posted something on the same page you're posting on, then you can just have their username. Probably no special formatting is necessary.
Otherwise, you should simply linkify their name to their profile. Beyond the URL, no special formatting is required. (For example, nutty about natty.) This is the recommended way to cite a user when reusing all or part of their post outside Stack Exchange, so it's probably quite appropriate here, too.
Please note that it's often at least as important, for clarity and usefulness, to link to the post (question, answer, or tag wiki), comment, chat message, or whatever other content by the user that you're referring to. This is not always necessary (though you should always do it if you're actually reproducing part or all of their work, or otherwise making a derivative work based on their work). But it often makes sense, even when their post is on the same page. Often users says "the first answer" or "the second answer," not realizing that the order answers are displays changes, often rapidly, and even appears differently to different users at the same time if they select different orderings (e.g., newest vs. highest voted).
The other thing to keep in mind is that if you ping a user, they may be notified. So this is sometimes beneficial. See How do comment @replies work?. Please note that notifications work in questions, answers, and edits to questions and answers--not just in comments (but, like comment pings, only if someone has already spoken).