Click Firewall panel, then Expert tab. Click Add. In the Add Rule dialog box, enter the following

Rank=1
Name=Blocked Site
State=Enabled
Action=Block
Track=Alert and Log
Source-Click Modify button, select Add Location, then click
IP Address. The Add IP Address dialog box displays. For Description, type Blacklisted, then for IP Address, type the IP address you want to block. Click OK. Note you could also click Host/Site and add the hostname, such as www.yahoo.com, instead of the IP address.
Destination-Click Modify button, select Add Location, then click My Computer.
Protocol-Click Modify button, select Add Protocol, then click Add Protocol. In the Add Protocol dialog box that displays, click the down arrow for Protocol, and click TCP & UDP. For Description, type Blacklisted. Leave Destination Port and Source Port as Other/Any. Click OK twice. Don't forget to click Apply in the lower right.

Hope this helps.

(tip contributed by) WATCHER Senior Contributor