I have a 5 user peer-to-peer LAN and use the RF102S to share dial-up Internet access among the clients, Can I completely block one (or more) of these LAN machines from access to the WAN (Internet) while leaving the remaining clients with full access?

Yes, here is what you need to do:

1) Click on filter settings

2) Enable BLOCK filter

3) Packet defined by = TCP/IP

4) IP address = ip address of the workstation you want to filter

5) Netmask = 255.255.255.255

6) TCP/IP service port = 0

7) Privilege Level = 1

8) Click on Insert

9) Click on OK and click on Save&Exit