[AccessD] The Polyp Problem

I know this has been discussed before, but I sort of removed a polyp
from my client abuser list last night, as a woman has the right to flip
out on deadbeats.  That is the law.  Here is the story.  Client
contracts for a job; agrees to pay whatever way - some do in stage I,
more in stage II and the rest in stage III.  It is clearly stated that
changes to the requirements of the system will be discussed and
additional invoicing will be required.  Polyp continuously *forgets* to
pay invoices as that is not is department, makes wild changes to the
system - "Oh, didn't I tell you?  Truck A, B or C can not go on streets
with a 2 Ton Limit?  You can just program that in, right?"  Or emergency
call - finger nail bimbo's system won't work and it is the hub.  Your
system broke it, we can't function, come over here right now.  Drop
everything, run over, and low and behold the cable is unplugged.  Three
hours out of your day, gee thanks.  Oh, we can't pay you, it has been a
bad year.  And that $2000 we still owe you from August?  That is coming
soon.  Hello, it is snowing!

In my warped world, I would like to put code in the program that when a
payment is not received, the system stops working.  When the bill is
paid, the user can have the encrypted password to keep working.

Doesn't that sound easy?  One final password when the system is paid in
full.  I know a geek could break into it and get around the password,
but these people are cheap to begin with if they won't pay and not work
continuing working for anyway.  Ideas?