What would result from putting a stop payment on a check for the down payment on a prepaid service?
Several things could happen, but the most likely is:
The company you gave the check to will come after you for the amount of the check plus additional fees.
Presumably you signed some kind of contract for this service, so if they do have to take you to court, they're likely to win. The contract probably has something allowing them to charge you for all fees incurred in collecting, so you'll also owe legal fees, probably of several thousand dollars, in addition to what you owe according to the terms of the contract itself.