How do I get my Customer to stop demanding and injecting free-consulting works within ongoing work agreements, dangling future carrots as bait, as well as ransoming my current agreement payments until I agree?

I have this Customer who has been with me for over 3 years. But one really annoying behaviour he has is that he continuously injects extra work efforts on top of existing agreements, and he keeps ransoming my existing payments.

Explaining the issue to this Customer does not help – he just can’t be bothered how he impacts my cash flow.

The business and funds are good for me, so I don’t want to break the business relationship, but I need this Customer to change his practices and behaviour towards me – how can this be done?