With the places you access the computer from do you use dynamic or static IP addresses? If they're all static IP addresses, try to block everything on port 22

With the places you access the computer from do you use dynamic or static IP addresses? If they're all static IP addresses, try to block everything on port 22 except for your static IPs. If it's dynamic you should have a pool address block, you can add the block in but there's a risk that if your block is in an ISPs home user pool that the botnet could infect people in your block. If you can't use upstream filtering then I'd suggest you configure a firewall to do the same thing, either in hardware (preferable) or software.