Usernames with "@" character cannot be deleted or merged
Fragment of a discussion from Extension talk:User Merge and Delete
Since 1.15 or even earlier the @ character is used at Special:UserRights to edit user rights on other wikis (see $wgUserrightsInterwikiDelimiter). It is also set as invalid character for usernames in $wgInvalidUsernameCharacters. Try unset( $wgInvalidUsernameCharacters['@'] );
