Wannabe,
Moral issues aside, it's simple enough on initializing the project, to check the system time, and run an 'if' statement on it. You're essentially forced to update the software by the date you specify however.
if now < SomeSpecifiedDate then
continue
else
msgbox " OMG wannabe has m4dskillz you shoulda kept him!!!11one one"
end if
SomeSpecifiedDate can be a registry setting or license file in the install folder.
Looking at the ethical side of it, I would heed the advice given so far. MP, Luis and Daniel have real world experience with it, and it sounds like it's probably not in your best interest.
I would say instead, that you keep the installation files and never turn those over. Your application will work on existing installations, but be a PITA to transfer manually. If you leave, your application will most likely die with the upgrade.