We use the DenyHosts package to help protect your account from brute-force SSH attacks. If someone fails to log in as a particular user several times in a row, the original IP address will be blacklisted in order to prevent additional attempts within a certain time period. If this happens to you and you try to log in again, you will see something similar to this:
ssh_exchange_identification: Connection closed by remote host
The blacklist expires IPs after a predetermined period of time. Typically, most users will not be affected by the blacklisting, but if you are, you will want to file a report.