The rules say a pilot has to be "available" for junior manning. If for some reason a pilot mistakenly answers the phone call from scheduling without thinking and CS states he/she has been junior manned, all the pilot has to say is: "I'm sorry, but I am not available for the junior manning assignment." Simple as that. As was indicated before, no explanations, no bargaining, no pleading or begging. The reasons you are "not available" to work on your day off may be plenty but they are not the business of crew scheduling.