On the one hand it's annoying people reply like that, but there's really not much that can be done to "fix" it.
What if they're attempting to reply to a post that wasn't the latest post by the user in the thread? You'd end up with even further confusion in the thread because of incorrect quoted posts due to the automated system.
@Onimua - it can be annoying I agree. I can't really see any usefulness in having the @username to get your point across to a specific user when the quote does the job.
I'm assuming the person your @ at would get an alert if the @username was used. What happens if a user picks a name that regularly get's mentioned this could become irritating if the user gets an alert wondering why they got it when it bares zero relevance. Or perhaps the alert kicks in if the @username is present in that thread in which case the alert to @username kicks in.
My people use @username to just respond to username regarding a post in this thread. It is possible that username posts on page 3 and the @username post is on page 8. For everyone reading the thread, it would be much easier if there would be a reference to the usernames post on page 3.
As forums are topic related, I don't want to know more about the username (linking to a profile) but I want to know what username actually said in this thread about the threads topic. The reply is always because of the idea mentioned within the same thread, not about a general persons position. Therefore I suggest to put a link or ideally a popup of the usernames last posts. Popups like the thread title popups in the forums.