I’m not in favor of telling her to go away forever – the problem is nobody’s told her that her behavior was slowly sliding into problem areas, then becoming completely inappropriate. It makes it hard to tell the person off, so I’d recommend just starting with a clear boundary line and going from there. I know some people who are COMPLETELY oblivious to signals, or even social norms – she sounds like that. Tell her clearly, directly, without anger and you might turn her into a good customer (of course, you might not).
Take her aside and say “Hey, I need you to stay out of the cages, the backroom, and the counter areas. I’m sorry I haven’t addressed this with you before, but those areas are for employees only.” No stares, no half measures – direct her not to do that. Do not debate this, it is not for her to review the reasons. Just say “Sorry, you can’t do that anymore. Please don’t.”
To add to this, if she’s still loitering after losing her ‘special regular’ status (that I’m sure she thinks she has), it’s time for another chat. “I appreciate your business, but the length of your stays in the store are becoming disruptive to me being able to run my business. I’d appreciate it if you cut back your visits a bit. If you need help with a purchase, let me know, but I’m afraid I can’t spend as much time on personal chats. Sorry.” No sighs, no signals – be clear and forthright. Then follow through.
You want to emphasize that she is a CUSTOMER, not a friend-of-owner, not a sorta-employee, not a volunteer, not anything special. You are staff, she is a customer, that is your relationship. If she keeps starting up chats with you, give her no special time privileges. A couple of minutes then find something else to do. If she follows, say, “I’m sorry, it’s nice to talk to you but I need to take care of some things. Do you need help with something first?” Keep her straight on in customer, not buddy, zone from now on.
Any guff, whining, being difficult, or she does it again anyway? Then it’s time for a “please don’t come back”. That sucks, but it’s in her court – she can choose to do as you’ve reasonably asked, or she can leave.