You need to setup the "URL content filter" on the Firewall tab. Use the "Enable URL Access Control" section and enter the URLs or partial URLs that you want blocked eg ebay will block ebay.co.uk, members.ebay.com and so on.
To exclude certain IPs from this filtering you will firstly need to create two "pretend" subnets and bind the MAC addresses of the trusted PCs to the first subnet and the untrusted to the second. For example your LAN is 192.168.0.1 255.255.255.0 - even though we are not subnetting it for the puposes of the block we are prentending to. So the first subnet is 192.168.1.1-127 and the second is 192.168.1.128-254
You then need to enable "Enable Excepting Subnets" and enter 192.168.0.0 255.255.255.128. This will exclude from the filtering all addresses between 192.168.0.1-127
You can of course change the subnet sizes to suit your needs.