Question : Stop----Wireless Access Point

You do I stop a user in my network from broadcasting our LANs DSL?

I want to add a vendor to our network but I want to make sure that he does not add any bells and whistles thus threatening our security.

Answer : Stop----Wireless Access Point

The only way to stop them is:

1) Don't let them connect a wireless router to the LAN connection.  If the need more ports tell them to use a switch.

2) If you think they would use the internal WiFi as a router you would have to use another wireless device (like a notebook) to scan the WiFi frequencies for unwanted access points.  I would use netstumbler to scan.  http://www.stumbler.net/