I've had one of these at my old house. A neighbour offered to help with my garden and I took her up on it as I'd just moved in and wanted to get to know people. Next thing I knew she was round all the time, and borrowing things; food, clothes, books, jewellery, make-up tools, basically anything that took her fancy she would ask for. If you tried to refuse, she was very adept at making you feel you were being unreasonable, selfish or that you had a problem.
It took me years to 'cool' the friendship as she was always, relentlessly looking for ways in with exaggerated niceness.
You need to wave her off when you see her, always be cordial but cut her off with, 'Sorry, I'm just on my way out, or rushing to, etc.' I told her that I had a new job that was very demanding and that I felt if I stopped just even for a moment I'd drop too many balls.'...
This went on for years, and eventually I ended up moving because she was always there with her 'niceness', always ready to suggest that she could come around to help. I once feigned annoyance with her but she just disarmed me with profuse apologies. The last time I spoke to her, which was a few days before the removal men came, she complimented me on my boots but then asked what size they were and if she could try them on... It never stops.
Whatever you do don't underestimate for far they will go. Focus on shutting her down at every opportunity but in a respectful way. Good luck.