I'm also a support professional for a software company, and there are times when a customer wants to do something in our program that is so far out in left field that we would have to rewrite a substantial portion of the program to give them what they want.

While we do our best to show our customers as many possible alternatives to their impossible requests as we can, sometimes it boils down to "No" - even then we suggest they submit a feature request to our development team.