How can i prevent usernames from containing special characters?

Hello,

Here is code that allow to use only numbers and latters within username.
You need to put this code snippet to site.php file:
http://www.amember.com/docs/Site.php_file

PHP:

Am_Di::getInstance()->hook->add(Am_Event::GET_LOGIN_REGEX, function (Am_Event $e) {
    $e->setReturn('/^[0-9a-zA-Z]+$/');
});

Also you need to change error message 'Username contains invalid characters - please use digits, letters, dash and underscore' accordingly. You can do it either at:
aMember CP -> Utilities -> Edit Messages



or within form configuration (click labels on username block)
aMember CP -> Configuration -> Forms Editor



Best Regards.