[artas_bartas]

In the past, I used several different strategies - it all depends on what type of client you have.

For greedy clients - give them a discount that is only valid if they pay up to a certain date.

For corporate clients - threaten to go online with a blog post/ social media story about how they screw freelancers.

For callous clients - yeah, delete the code or switch on a few unexpected "extras". The funnier extras, the better.

It's a good thing that you have a contract, though, should come in handy if things turn out nasty and you'll need a lawyer.