When I started my career, I started working corrections in a state prison and than a country jail. Jails/prisons are full of these nut jobs. They talk and talk and talk some more and its generally hard to get a word in as they babble on and on about useless drivel not making any sense.
Basically, you need to do a few things. Step one, stop being nice. They like nice people because they can babble on and on. Learn to be rude and short. It's hard to be rude to a person because we are ingrained to be polite and nice to people in public, but some people understand rude and get the point better.
Step 2, is to talk loudly and forcefully (raise your voice so you sound serious) and to direct the conversation as you want it to go. You need to break their thought processes so that the "conversation" can go two ways and not them babbling for 30 minutes about nothing.
And absolutely don't be afraid to have the cops escort the nut job off the property if she wont leave. I can guarantee that the local cops already know this individual very well.
Like I said earlier, take the kid gloves off when dealing with this type of person. Stop smiling, stop being nice. If you see her pull into the parking lot, I wouldn't even let her get out her car. Approach her, ask her if she is their to pay her past due bill, and tell her that she is no longer welcome as a customer, please leave immediately, and have a nice day. A 30 second conversation at most.