we had a similar problem here, constant attempts to guess the systems password filled up the log to epic proportions.
you could disable external access via the windows firewall (if you're using windows SBS 2003) its fairly trivial but the end solution we opted for (because we still required external access ourselfs to the database server) was to change the default access port to SQL server; it seemed to stop alot of problems.
although if possible, I would also considering changing the architecture of your network slightly (sometimes this isn't possible if you have purchased some virtual machine from a service provider); moving your database server and disconnecting it from your hub/switch and plugging it into the back of your web server (if this is the ultimate use for your databases) so the web server acts as a type of proxy, prevent all external internet access.