Terms of the contract may spell out cancellation options. As the seller, there are situations that allow for the cancellation of the contract. Speak to the real estate agent for specifics in your state and with your agreement.
It depends on the specific language in the contract. Generally, when the buyer is out of their contingency period, the seller can force them to comply or cancel.
The seller of a property can cancel a contract only if the buyer has breached one of the conditions contained within the purchase agreement. It is the seller's responsibility to then inform the buyer that the breach has taken place and the contract will be cancelled immediately.
The contract should have conditions outlined that would provide for either party to be released from the contract. Read the contract very carefully to make sure that you can be released without any obligations.