|Yep. Some sort of internet filtering is definitely needed. |
If you feel the need, and I would recommend this although for your network it may be overkill, get a good firewall/router. Most Linksys/Cisco, D-Link, and Sonicwall devices can block access to all of the internet except for certain sites that you choose. Sonicwall also has other good blocking features. Because these are hardware firewalls, they will only affect the machines when they are connected to your local shop network.
Another option is to add an additional firewall to your network in addition to whatever hardware firewall you are running now. (You ARE running a firewall/router, right?) In this case it would look something like this:
Main Router (normal firewall rules)
Local Customer (Custom blocking rules)
Workstations Firewall & Router
What this setup does is it allows you to set up custom blocking rules for your customer computers without impacting your local workstations. Because even $39 D-Link routers support custom web filtering, this may be a better and more cost effective way to go.
Let me know if you need more info.
IT Desktop & Network Consultant - MOS Master Certified, MCP, MCSA, MCITP - Windows 7, CCNA Certificate Pending, A+, Network +