Take e.g. the username t... Maybe they do it on purpose, but the @reply-system is there for a good reason.
Further to my comment above, which I can no longer edit, see Jeff's answer here. He states that case insensitive match of at least THREE characters for the reply to work. So as this users name is 3 chars it should work fine.
You don't have to be able to use
@name to reply to their comment. Even if the name is too short (can you do that?) you just quote what you can. If the user doesn't get the notice that you've made a comment about their comment, that is their problem. Now, if you are really asking for a mechanism to be able to contact a person reliably, then that is another issue - rather independent of the being able to respond to them via comments (which is just a convenient trick in the absence of separate communication). It has been requested before (back in the days before MSO, let alone since). I still support that request - but then people who want to contact me can by looking at my profile. I can understand some people not wanting to be bothered by people contacting them out of the blue; so far, I've not had anyone abusing my contact information. And I'd ignore abuse - or point out that it is abuse the first time and ignore it thereafter. Of course, Gmail makes that easy if I need it.