Call 866-347-3292 to cancel.  

Keep in mind that you may be subject to an Early Termination Fee.  http://legal.hughesnet.com/ServiceTermsAndConditions-current.cfm

"If you cancel after activation but before the end of your 24-month term commitment, you will be charged a Service Termination Fee of up to $400. The exact amount of the Service Termination Fee will be $400 for the first ninety (90) days after activation of the HughesNet Service. Thereafter, the amount will decrease by $15 per month for each month of active Service."

You'll also be required to return the modem, power pack and radio from the dish.  They will send an equipment return kit when the account is cancelled, which is normally at the end of the current billing cycle, unless requested otherwise.