Blocking traffic between workstations

We have had a request come up to block all traffic between workstation endpoints.  We want to restrict the endpoints from talking to each other but still allow the endpoints to talk to servers.

Is there a way to do this using SEP 12?  If so what are the best practices for implementing such a block.  We are mainly using Windows 7 endpoints and Windows 2003 and 2008 servers.

Thank you