Abusive customer? Philippines

Hi! I need to know what will you do if your customer is beeing to abusive of your service? Like for example they wanted something more than what the contract says. They don't want to pay anything for additional item/service they're asking, and if you refuse them they'll get upset. Or that they'll be thinking 'pineperahan lang sila'? They also demanded so much, wherein they've been asking rush service although, again, they're not paying any additional payment for said 'rush' service. What to do?