how to reject users which try to login w/o client certificates

Hi! Question: What is an easy way to reject users who are going to connect from a machine which does not have the appropriate client certificate? Note: I'm talking about special users only. Background: At school we have a bunch of electronic whiteboards with WLAN. All of them have the same username/passwd with client certs installed. Just to be on safe side I like to make sure that nobody else is abusing this username/passwd from another device. You never know.... Thanks a lot Uwe