How to handle client requesting free work after signing agreement?

I have taken a contract for an interior bungalow, but the client is always asking for different designs and extra work for free. I have an agreement, and I have given all designs to them. After the agreement was signed, work started, and they are now saying they’ve paid too much and asking for free work. They are arguing about the furniture and the entire work, saying it’s not what they expected, even though I provided all the designs in advance. How can I handle this, and can they file a case against me?