Removing TSR's

I have recently started programming TSR's in C. can anyone tell me how to write some code that will unload the TSR from the memory if the user enters a specific word.
and yes, one more thing, when i invoke my TSR from a DOS shell, it remains effective only for that particular shell and does not affect any other instances of DOS shell that i might open and it ends as soon as i close that DOS shell. can someone tell me how to fix this.
hope i have defined my problem clearly. if not, then pls lemme know, i'll try to define it somehow else.
thnx