Hiding the source isn't going to stop the client from cancelling a contract or continuing to run the software without you.

It sounds like your question is more about how to keep a client giving you more money than anything technical. Instead of that, sell the software installation for a price that makes you happy even if they don't give you any more money.

If you want them to give you more money, improve the software and add features which make them come back to you. :)