If someone tries to register from an IP address that a silenced user
has registered from, prevent it.
When silencing someone, silence everyone else who registered from the
same IP address.
We throttle registrations by IP. We record IP address of each
registration, and if too many registrations have been done by the same
IP address in the time interval, we reject the registration.
Siebrand Mazeland
Brion Vibber
Evan Prodromou