It depends on whom you want to say these bad words to. If it is to a friend it's best to say, "No I don't like this/ don't like the way you treat me/are talking to me"or "No it is not possible for me to ................" or" No it's just not on" or "Sorry I'm too busy this weekend"
If it is your employer, say, "I would love to attend to your request, but it is not possible at present because I am working hard to finish Project X/ Report Y for our company".