She has long harboured grievances, all of which have been shown to be without substance. How on earth are we to get rid of her?
This parent may well be committing an offence under the 1996 Education Act and a letter to her pointing this out might persuade her to desist. You, or the clerk to the governors, may also write to her, forbidding her to enter school premises without an explicit prior appointment.
A home visit by the education welfare officer might be another way of making her realise that her behaviour in the school is unacceptable. The officer might also be able to talk to her about her grievances and provide her with some reassurance.