To block access to one site you can open IIS Manager>go to "web sites">right click on the site you want to edit>click "properties">click "Directory Security" tab>Click "edit" button next to IP address and domain name restrictions>add the ips you want to block. You can block just one or a subnet.
Caution!!!
Read and know what you're doing or you can lock yourself out!
Use the help file or search Microsoft for more info.
Or as Gris says do it in the firewall.