Your suggestion would require that the way comments are implemented be changed. Currently, the @reply system attempts to match the first three characters against the list of people who have commented on that post. It notifies the most recent commenter that matches (and the post author). It does not save that information, which means that if a user changes their name, it is impossible for the system to determine who the @reply was for.
That being said, I'm still against your proposal. If I post a comment in the heat of the moment, or post something laughably or rudely incorrect, I want to be able to delete it. Especially if someone else comments on it and points out the error, then I want to be able to remove the offending or problematic comment. I should always be able to do that, because if it is bad for SO then I should be able to get rid of it. Especially items that make me look stupid, if someone could just type "@Devin" and then lock my comments in permanently, that's not really fair.
I think it IS a reasonable suggestion to notify those who have @replies that a previous comment was deleted. But as mentioned earlier, with name changes and name collisions, this is nearly impossible. The other side is that sometimes people have conversations in comments without the @reply syntax, so they would still be out in the cold. Unless you notified everyone on the comment thread after the comment. And that would be annoying.