>>When I was in the consulting business that is how I wrote all of my apps. Also, when clients were on a pay as you go plan I added a password feature that kicked in after the software was supposed to be paid for. If they hadn't paid by the due date then I didn't phone them the password and as soon as the "in arrears" message popped up and dropped them back to the OS I got the call, and the check.
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>I already ask for the password, but then they're told what it is (or it's configured for them). Another thing is configured too: the s_demo system variable - it's set to 200 or 5000 or whatever we think they'll use up by the paytime. They get an alert saying "the free evaluation version of < app name here > has exceeded its limitations. Contact Alas LTD at ...< phone here >". It fired just a couple of times so far.

I had one client complain that the program wasn't any good and they had stopped usinging it, which is why they didn't send the last payment. When the three month time limit expired which was about two months after the complaint, I got a call. They had been using it all along and wanted to continue using it. "Could you get it going again?" Sure, right after the check is in the bank.