Don't forget, even if you make it so only certain classes can log-in, users will ALWAYS be able to "create" them - if they're in that client game version... Because there's pretty much NO way to MAKE someone use your actual/full client, so even if you removed them from a client you distributed; they could still essentially use their own client, with certain files (from yours) to still be able to connect to your server (like gshop and the DATA files [ele tasks etc]), and STILL be able to "create" them. May not be able to log them in, if you have your configs right, but (ijs) there's NO way (that I know of) to actually stop their creation...
Example: you ran a v146 server, but ixnayed the sin/psy/seek/mystic; you could remove them from your client too - BUT, a user could just login using their own client, with your data files, and still be able to create a sin/psy/seek/mystic (even though they will not be able to log it in)...!
Not too sure why this would be a real *problem* though anyway, unless you're concerned about any "space" (read: HDD/storage space) they may be "wasting" (by creating a toon they can't even log in)...! But I can assure you, any of this "wasted space" will be pretty damn minimal! I recall doing a test a LONG time ago, to see how much psychical HDD space a toon/account could take up (no I'm not digging it up)...
I could understand the desire though, because then at least your "product" looks and feels professional by not having classes you can't actually log in, available to "create"... It would be a nice "polish" or "professional" appearance - for distribution / use... You'd just have to bear in mind, if for some retarded reason someone wanted to, they would still be able to "create" a non-playable class using a "stock client" with your data files...