Use Open Source software to defend against spyware

Here's the idea:
Configure squid to block Internet access for Internet Explorer. Configure the firewall to require that all Internet web browsing go through squid. Looking at the squid documentation I see that the FAQ lists that ACLs can filter on the "User Agent" value from the browser. By blocking IE (and Outlook which uses IE to retrieve web content) from accessing all but approved sites, you could virtually eliminate adware / spyware issues from the network.

Good idea?