The number to cancel service is 866-347-3292.
If you are still under contract, you will be charged an early termination fee, as per the service agreement (see legal.hughesnet.com). You will also have to return the equipment or be charged an extra fee for that. The company will send you a box and label for the equipment.
The service agreement contains information about how to initiate a dispute with the company if you wish to follow that route.
*I am not a Hughesnet employee or representative. This is a customer-to-customer tech support community, and I am a customer.