[final kaoss]

look in the user registration options in your admin cp. You can toggle the settings so they can't use those characters in a username

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
^((?!&#\d+[\x20-\x7E])+$ - ASCII characters from 32-127

See PHP.net for more information on regular expressions.