If an ATC clearance has been obtained, there is no pilot that is allowed to deviate from the given clearance unless an amended clearance has been obtained, there is an emergency, or the deviation is in response to an air traffic alert and collision avoidance system resolution advisory.
However, unless you are in a Class A airspace, a pilot is allowed to cancel an IFR flight plan if the operation is being conducted in VFR weather conditions. If the pilot is unsure of an ATC clearance, they can simply request clarification from ATC. Unless there is an emergency, no person should operate an aircraft contrary to an ATC instruction.
As well as this, any pilot in command that deviates from an ATC clearance in an emergency, or in response to a traffic alert and collision avoidance system resolution advisory should notify the ATC of the deviation as soon as possible. A pilot will be given priority in an emergency situation, and they will need to submit a report of this emergency within 48 hours to the manager of the ATC facility if this is requested of them.
Unless it has been authorized by the ATC, no pilot should operate an aircraft according to any clearance or instruction that has been issued to the pilot of another aircraft.