The spambot and its posts have been deleted. Thanks to the moderators who tried to deal with it earlier. Unfortunately the stock flood protection leaves a lot to be desired.
There's an old and unsupported phpbb mod I'll try if it occurs again - don't want to install it immediately in case this is a one-off and the mod causes more problems than it resolves.
IMO, a good idea is adding a usergroup for spammers (which drops its users' privileges) so a moderator would at least have the ability to suspend a user temporarily until all mods agree to ban him (or not).