Need advice on handling needy clients

I have a client who asks for way more than what they pay for. They frequently ask for custom development work that goes beyond just basic functionalities and want completely custom flows for their business. It’s difficult to manage and yes, I’ve already used the line of “I’m happy to accommodating work beyond our agreed upon scope, but it will come at an extra cost”. However, it’s becoming a monthly issue and they doesn’t want to revise their contract/sco