Let's face it: there are a lot of assholes out there. Between people who can't handle their booze, inexperienced players, and those who think the world revolves around them there are definitely enough to go around. If you're going to do any form of public bondage or domination, you have to be prepared to deal with them which can mean taking on an intimidating demeanor.
When you've got a boy incapacitated or subservient in public, you should view every person in that room as submissive to you to some degree. That boy is your responsibility, and anything bad that happens because someone doesn't listen to you is still on your hands; a boy shouldn't have to come out of headspace because you can't keep a creeper from groping him. It's really easy to want to be overly polite in social settings, but in this circumstance either a line is being crossed or it isn't. A firm (but not outright rude) response to inappropriateness ensures the conviction surrounding your warning is well-received.
Be aware of management, bouncers, bartenders, or similar. If you're in an setting that allows for an - even casual - public scene, odds are that the staff is at least remotely aware of proper etiquette surrounding the situation. Even in the event that they're not aware of the specifics, "No means no," is a pretty universal code. If someone remains unresponsive to your most adamant of reprimands, don't hesitate to inform official staff that you're having problems with a given person.
It's also useful to have trusted friends around. Not only is there strength in numbers (on top of helping subdue a boy) when it comes to people hell-bent on interfering, but they can help in notifying staff of problems. If you've got a boy tied to a St Andrew's Cross and someone keeps trying to spank him after being warned several times, you can't exactly leave him there and walk off to inform the bouncer.
Just remember: alcohol or not, it's really not that hard to be considerate. If it's a very sexually charged setting, at least be patient enough to allow a given person one freebie. When you engage in public play it's always a risk that someone might misread body language, expectations, interest, etc. and overstep, but persisting after initial reluctance is not okay regardless of the circumstance.