Look up their IP adress and enter it as a blocked IP in your firewall.
But, most people get allocated a number from a block when they connect to their ISP. So their IP address may vary. In that case you may want to bar a "block", say 123.45.67.01 - 123.45.67.255
But, that may also bar people who you want to allow in. (Person X who you want to bar may be with say BT. Person Y who you want to allow may also be with BT)
Once you get used to tweaking the Firewall, you can just ban them almost instantly and then re-enable access for the good guys.