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

[uj2.hahn at posteo.de]

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