1. Go to Firewall policy > Rules.
2. Click on Add Rule button. Select Host > Next > From Address Type drop down menu select DNS domain.
3. Select DNS Domain as *.* then Click Next > Click Finish.
4. Once the rule is created, highlight the New Rule. Go to Service column, right click and edit, then select Add. The rule will be TCP, Source/destination with remote port 80,443 click ok and ok again. Then go to Action column and make it set to "Block".
The above rule is to block all the websites.