If its on the menu or posted ahead of time, you can bring it up as an issue with the manager - but if they refuse to remove or adjust it, your only option is to leave the restaurant without ordering - you can't refuse to pay it after the fact.

If it shows up on the check without prior notice, you can refuse to pay it or request it be adjusted to reflect what you want to pay - and if the manager refuses, my advise is to call your credit card company (Amex is best for this), explain the situation and they will advise you how to handle the receipt to make the dispute process easier. If you're paying with cash, you can settle the check and your preferred tip amount and just leave - if they make a scene, you can tell them to call the police....the police are not likely to backup the restaurant if the service charge was not disclosed ahead of time, nor are you under any obligation to sit and wait for them to arrive.

Keep in mind, some of these restaurants in Miami (especially the ones along Lincoln Rd, Ocean Dr, Washington and Collins that use girls to lure in unsuspecting visitors) have rather "hostile" management, and you can expect a rather heated screaming match - some of those people are completely batsh**.

Don't be afraid to stand your ground and walk away - but always look for posted policies (gratuities and per-person or table minimums) before ordering. Most restaurants will enforce a gratuity for larger groups, but again, if you feel the service was not up to standard, don't be afraid to tell the manager and request the tip be adjusted.