This is a mid-90's issue for ISPs.  I'm not a huge ISP guy
(did infrastructures not edges), but AFAIR, Radius was sortof
intended for this.

Can you get your services to use a single authentication source
(radius, ldap, what have you)?

And you might just remove all accounts that are not paid for.

Quoting Martti Kuparinen (martti.kuparinen@iki.fi):
> Let's say I have a small ISP with dialup lines. Users can dial in to get
> Internet access but they can also use our services (webmail, ssh, ftp)
> from the Internet.
> 
> I want to know when each user has logged in (dialup, webmail, ssh, ftp) last
> time in order to know if an account has not been used for a while. As an
> example, I want to remove all accounts which have not been used for 6 
> months.
> 
> What is the easiest way to gather this "last login" information in to a
> single place (maybe MySQL database)? All ideas are welcome...
> 
> I'm using NetBSD with MySQL, Radius, SquirrelMail, etc.