For contracts that have not been signed you can simply choose to Delete or Archive the contract altogether. To do so, simply hover your cursor over the contract tile until the (...) icon appears. Clicking that will bring up the Delete or Archive options.
If a contract has been signed by your contract, you'll need them to agree to cancel the contract before it can be deleted. To request that from them, simply open the contract from your Dashboard and select the 'Request to Cancel' button. Doing so will send a request to your client, they will have 72 hours to agree to the cancellation.