Be polite firm.  You’re not required to explain every decisionyou make.

    If you must give a reason, point it calmly and sensibly.

     Don’t be afraid to ask, “What should I?” or “Is that fair?” If people can’t give you a good enough answer to either question, they’ll see your point and leave you alone.

     Stand your ground.  Don’t say, “NO” then back down later.  People might think you aren’t firm in your decisions, and might store that knowledge for future manipulation.  Good luck!