You can only delete an agreement that hasn't been accepted by your customer. To do this, go to the customer record, click on 'edit service agreement' and you will see the option to delete the agreement from this address
If the agreement has been accepted by your customer, you will only see the option to cancel the agreement.
You will be asked to confirm the cancellation of the agreement. All, if any, future payments will be cancelled and your customer will be notified that the agreement has been cancelled.