"Is it OK to not accept an ATC clearance? What should we say to the controller in this case?" - Charles S.
“If ATC issues a clearance that would cause you to deviate from a rule or regulation, or in your opinion, would place the aircraft in jeopardy, it is your responsibility to request an amended clearance.
Pilots have the privilege of requesting a different clearance from that which has been issued by ATC if they feel that they have information which would make another course of action more practicable or if aircraft equipment limitations will not allow compliance with the clearance issued.
If time permits, simply request a different clearance from the controller. As in any negotiation, it’s helpful if you include your preferred alternative and reason. For example, if the controller issues a runway that has too much crosswind for you, request a different runway. You should state—unable runway three six, request runway two seven due to crosswind.
Whatever course of action you decide upon though, you are expected to inform ATC accordingly. You are expected to cooperate so as to preclude disruption of traffic flow or creation of conflicting patterns. You are also expected to use the appropriate aircraft call sign to acknowledge all ATC clearances, frequency changes, or advisory information.
Don’t get into a prolonged discussion on the frequency. Controllers are not able to take time from their immediate control duties nor can they afford to overload the frequency to furnish explanations. If you want an explanation of the handling of your flight at the time of occurrence; call the facility after landing. Phone numbers are listed in the Airport Facility Directory. Flight Service can get facility contact information as well.”