It's possible to block an IP-address from your website. Go to System > Configuration, and select the Security settings tab.
Click on Block IP-address to open the IP-address blocking system.
Blocking an IP-address
On the page of the IP-address blocking system, you'll find an input field where you can write down the IP-address you want to block. Click on Block to block the IP-address.
Unblocking an IP-address
It's also possible to unblock a blocked IP-address. Click on Unblock at the end of the correct row. The IP-address will be unblocked immediately.