Special Characters in Usernames -- Can I Disallow?

I've been googling all over for an answer to this; nothing. And since this is the friendliest and fastest WP community on the 'net, I'm going to bypass all others and ask you lovely people. :slight_smile:

I have had one user put an @ at the beginning of his username. This looks horrible everywhere on the site -- like @@username.

Ideally, I would like to stop users from using anything other than a - or _ in a username, but I'd also be OK with stopping all non alphanumeric characters. It seems you can do this in Wordpress (default) but in Buddypress, it's default to allow it.

I use Buddypress. :stuck_out_tongue:

So....does anyone know of a way to control this? All the plugins and threads I've found center on adding the ability to use special characters, not getting rid of it. I'm somewhat familiar with the "sanitize" functions, but I'm not sure where to start since WP and BP have different rules.

And at present, I've got one subsite where registration is being controlled by WP and another where it's being controlled by Buddypress. Yes...I will merge them in time. But for now....

Is there a WPMUDEV plugin that I haven't yet discovered, or a security plugin out there somewhere (I use WangGuard and iThemes Security, neither seems to have this option) which would provide the ability to block certain characters at registration?

Thanks in advance for any advice!