IWTL How to have my time be respected.

I know I cannot control others, but would appreciate if anybody would let me know anything I can do to have others respect my time. For example, often I get told things like "well what are you so busy with" or "you don't look like you have anything better to do" and I don't feel like should have to justify not giving my time for whatever reason. Is there some way I can encourage these types of people to respect my time w/o just faking it and pretending to be busy?