From: Northern California
Under Policy Elements then Destination Sets, create a new Set. You'll need to put in the destinations you want in the set. When you do, put them as *.domain.com. So, you'd put *.hotmail.com and *.gmail.com, etcetra. Once you have created the set and given it a name (such as Banned Webmail Sites), go to Access Policy then Site and Content rules. There, create a new deny rule that denies all users to your Banned Webmail Site.