I'm not sure what your question is. I think the answer depends on what you mean by "if they didn't want to pay the 18%."
If there was an issue with service, the thing to do would be to speak to a manager and express your concerns. If your complaint was reasonable, I'm sure they'd have no problem making an adjustment in the gratuity. (Please keep in mind, however, that a LOT of the service issues we have in restaurants are not really the fault of the server.)
OTOH, if someone simply didn't want to pay the 18% because they thought it was too high, or they didn't agree with the concept of a mandatory gratuity, I think they'd be out of luck.