As suggested, review your deposit contract for language that gives you the unilateral right to close the account for a reason or reasons of your choosing.
This is not a circumstance covered by UIGEA, but the activity may still be illegal. Also, if you feel this is constitutes financial exploitation of the elderly, you may consider any reporting responsibilities you may have to the state or in connection with SAR filing.
You can't stop her from carrying on this activity at the next institution, but you can at least say that you did everything you could...
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.