Right now, we are apparently returning a 400 if a user uses an empty or generic user agent.
According to https://tools.ietf.org/html/rfc2616 , 400 is "The request could not be understood by the server due to malformed syntax." whereas 403 is "The server understood the request, but is refusing to fulfill it.". We understood the request but are refusing to fulfil it because we don't like the user agent string, so 403 seems more appropriate to me.

Either way, http://wiki.musicbrainz.org/XML_Web_Service/Rate_Limiting should also be updated. Right now it only mentions that users will receive a 503.