There are a few ways to do that.

1 would be a server logon trigger.

Another (better imho) method is to use an AD account. Then place login restrictions at the domain level for the account (http://technet.microsoft.com/en-us/library/cc781861(v=ws.10).aspx).

Another method would be via firewall rules (do you have your databases segmented from the rest of the network via firewall??).