My response:
I would imagine that their contract is used both in "home" solicitations, as well as at "business locations". You were at a business location.
Since you agreed to Utah law, the following is the controlling law in your instance.
Utah Home Solicitation Statute:
70C-5-101. Definition -- Home solicitation sale. As used in this chapter, "home solicitation sale" means a consumer credit sale of goods or services in which the seller or a person acting for him engages in a face-to-face solicitation of the sale at a residence or place of employment of the buyer and the buyer's agreement or offer to purchase is there given to the seller or a person acting for him. It does not include a sale made pursuant to preexisting open-end accounts, or a sale made between the parties at a business establishment at a fixed location where goods or services are offered or exhibited for sale.
Since the contract was signed at a "business establishment at a fixed location where goods or services are offered or exhibited for sale" it was proper of them to cross out that portion of the contract.
Therefore, there is no 3-day "cooling off" period for the contract you signed.
Since you signed a contract, cancelling your check will do you no good. They could, at the very least, have a bad mark placed on your credit record and, at worst sue you in Utah for the contract amount, interest at the legal or contract rate, attorney's fees, court costs, and other collection fees.
In summary, you're on the hook.
IAAL