The spammers were using unidentified nicks. The network has channel mode 'r', that is used for only allowing identified nicks into the channel.
I'm not really a fan of channel mode 'r' for support channels. Someone might join the channel for a simple question or two, they get the answers and never return. Having to register a nickname and check email can be a bit annoying for some people.
The network also has user mode 'R' that blocks messages from unidentified users.
They added a spam filter into the IRCd over a year ago.
https://freenode.net/kb/answer/channelmodes