To cancel, you will need to call 866-347-3292. You may have to pay an early termination fee if you're still under contract. See http://legal.hughesnet.com/ServiceTermsAndConditions-current.cfm To avoid equipment charges, you will also need to return the equipment (modem, power pack, and the radio from the dish). You will get a kit to return the equipment when you cancel the account.
Here's the illustrated guide to returning your equipment.