Hmm. I feel like I'm missing a bit of the situation, but I understand the need to change details to avoid detection. I guess my first suggestion is, don't spend your leisure time there, when you know Nasty Woman could pop in the door at any moment. I don't know how realistic that is, and it certainly isn't fair; but the best way to avoid someone unpleasant (as the subject says) is to simply not be in a location they're likely to be in. It sounds like, in one story, you were trying to leave as soon as you saw her enter, but as you related, it didn't actually work well because you had to walk by her in order to leave, and she tried to get you to assist her. As it appears there's only one exit you can use, which she would likely be standing by, the easiest thing to do is to leave before she shows up.
Is there any kind of front desk she would have to pass, such that someone could send you a quick text if they see her coming? Even if you don't have time to actually leave, perhaps you could duck into a back room, then skirt the edge of the room once she's entered and moved away from the door. It's a little Mission: Impossible but might work for the occasional awkward situation.
If you must encounter her while off-duty, I like the idea of saying something like, "Oh, I'm so sorry, but as I'm off-duty, I'm not allowed to do XYZ due to the liability. I'm sure someone else here can help you, though." And then walk away quickly. If it's just you and Nasty Woman, you could say something more abrupt; but the longer form might assure nearby patrons that you aren't being purposefully unhelpful. Also, of course, you should avoid helping anyone once you're off-duty, not just Nasty Woman, because otherwise it will become clear that you only pull out the "liability" card for her.
As it sounds like this is not a business that can bar Nasty Woman from being a patron, maybe your boss could discuss with all employees ways to deal with unpleasant patrons? It would probably be a good general plan to have, if everyone knows they can use the "liability" excuse, other on-duty employees are keeping an eye out for them, etc..