That is a good diagram! I'm not 100% sure this will work, but I think it will.
You can assign a group policy to a VLAN. So start by creating a group policy (Network Wide/Group Policies), lets call it VLAN10 (you can call it anything you want). Then go "Security Appliance/Addressing and VLANS", click on the VLAN you created, and select the group policy you created.
Then go into your new group policy and for "Firewall and traffic shaping" select "custom" to create layer 3 firewall rules. Then create "deny" rules to block traffic to the other site. Make sure you move the deny rules to the top, as the last rule is a permit.