Quote from Bob Smith :

I've asked Vic about custom titles, even though the option is there, apparently there's something odd about them, so they have to match up with a user group.
E.g. all mods say "mod", all admins say "developer", and all banned users say "no longer welcome".

So unfortunately we can't change them just like that, or I would have adjusted Tristan's to "forum spammer".

On vB there is a modual (not sure if you need to add it though) which allows custom titles. I remember the last forum I used on vB had it activated if you made ~50 posts. Of course there is always the hardway of making new user groups with different titles just for some people (but this is a lot of hard work for a silly title).