[Lynne]

I think you can go to vboptions > user registration options > username regular expression and make it so only letters and numbers may be entered. See the help button next to it:

Username Regular Expression
You may require the username to match a PCRE-type regular expression.

(Do not start or end the expression with an escape character)

Examples:

^[A-Z]+$ - Characters from A-Z only
^[A-Z ]+$ - Characters from A-Z including space
^[A-Z0-9 ]+$ - Alphanumeric characters including space
^[\x20-\x7E]+$ - ASCII characters from 32-127

See PHP.net for more information on regular expressions.