Yep, we use various tools to block spammers but occasionally they block innocent users too. Among these are worldwide 'blacklists' of known usernames, email addresses and IP addresses of previous spammers. If someone is constantly getting blocked it may be due to their choice of username or that they are using a VPN which has got itself blacklisted.
We also manually check every application before granting posting rights.
@Adam&Emma if your brother is still having difficulties PM me the username,
and email address he wants to use and I'll set him up manually with a temporary password which he can change once he gets logged in.
CT