ok, first, you dont need to post twice, this is a bad idea that only makes everyone grumpy and could get you negged.

Restricting Yahoo: assuming you are a admin at a company (comes off that way) they have a corportate firewall, Im sure. Block outgoing access to yahoos ports. (google for them, I dont feel like doing ALL the legwork for you.) THEN you need to find out what the site is for yahoos lite client (web access) and fix that in thier hosts file, so it does not resolve. You could also do it in the corporate DNS if they have one, or block it in a corporate proxy. Its tough to stop entirely, but thats how ID do it...