How to disable access to LAN via VPN Debian Gateway Server

I need to access the vpn password file on our linux debian gateway. I cannot remeber where I add and remove vpn users access to our lan. We just had the network admin quit along with some other key players who would access the lan from there homes. I need to disable their ability to vpn. I have changed all the admin passwords but would like to disable their passwords to the gateway. Please help.  Thank you, I think this is a easy problem, :)
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It depends on your VPN type. Is it PPTP, PPPOE or SSH tunnel?
It may be sasldb | chap-secrets |  pap-secrets files or ldap database or shadow file or rsa key file.
Gabriel OrozcoSolution ArchitectCommented:
I think you should check your /etc/ppp/chapsecrets file.

other vpn's do not use usernames but certificates
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