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.
The only time HughesNet throttles service is when you run out of plan data. That's it. That doesn't mean system load doesn't affect your speed, whether at your beam, your gateway or with the entire system, but at no other time than running out of data is your service throttled
If you're running out of data and need some tips to help prevent it, I suggest reading the following two posts...