Agreed Robinpud- the point about volunteers is that they are exactly that- and unless they have to go through a selection procedure, it is hard to say "no" unless there are good reasons for turning them down. However, if a teacher feels that a particular parent would be unhelpful in their classroom, surely they can just say that they don't need any more help just at the moment, but thanks for the offer?
I agree with your pint about people management- as someone who has worked in and out of teaching for my entire career., but also worked in commerce and re-trained as a counsellor, it saddens me that many teachers have poor social skills- (this is a general point, not aimed at inthegutter)- it is as if they can only relate to children, or when they are in a naturally superior/controlling position- and as soon as that is under threat in any way, they cannot adapt.
I think that in all these situations, people need to turn the situation round and ask what THEY are afraid of, and try to change THEIR behaviour. For instance, if a teacher feels "threatened" by a parent, then they need to question their own self-esteem and confidence as ateacher. If they feel that a parent may "blab" outside the school gate, then they need to ask if they are in need of help with their assertiveness skills, so they can manage that situation to everyone's satisfaction.
Often the problem is not the parent, per se, but the way that a teacher reacts to them - and that means examining their own beliefs, confidence and so on, in trying to deal with the situation, and perhaps confronting their own fears about not being in control.
it is acliche, but you cannot change other people- you can only change yourself and how you adapt to a situation.