personally I'd advise against removing db data unless you actually added the data yourself. even though you wrote the scripting behind the app, if they entered the data and you destroy that, the ball will be in their court when you get to court.
just knockout one of the main files (one that you wrote and own until paid for) if possible.
$killswitch = (time() > 12341323) ? rename('db_access.php','db_axess.php') : '';
// untested, should work tho. just renames the file whenever the page is called if a certain date has passed.
where 12341323 is the unix timestamp of the 'payment by...' date.
might want to call it something other than $here_be_the_kill_switch and just subtely rename an important file which you have script permissions for.