s = subscriber, paid member
u = unsubscriped, free member.
It's possible, of course, to use this service to register many names and launch a spam attack using it. But, in that case you can just block all `u<star>@<star>.irccloud.com` (assuming you can rdns properly)
Like I said, I'm not a super large advocate of irccloud, but just as webchat passes on the originating IP via cgi module extensions, irccloud fixes problems with abuse for operators.
I know this, because I have been running a large irc network for over 10 years.
: I tried everything to make asterisks appear, but hackernews is doing something funky:
You allude to a problem by suggesting blocking u*, these accounts can be created when the old one is blocked. Even subscriber accounts can be created at will & higher cost, though are likely less of an issue.
There is a problem that irc.example.net can't say "These users all seem to be originating from this single open proxy, lets block it." They need irccloud to do that.