Give them a test: if they are reasonable they will respond to anything reasonable in a reasonable way. So ask them in detail why they want to do this, how they want to do it, are there alternatives. If they answer in a fair and honest fashion, then propose some way to solve their problem in a way that is fair to you. There are lots of good suggestions here: third party, at your site, hiding the secret sauce, etc.

If they are bullies, irrational, arrogant or silly, then save yourself lots of headaches and very nicely say its not something you can do.