if you have power of attorney, surely you could set up a second account that she doesn't know about, and transfer her pension into this new account, and change the care home payments to come out of the new account, put a pittance of pocket money in for her, and if she goes to the bank, she'll soon realise she hasn't got much money to actually take out. if she questions it, tell her that the care home take their money out, and the chemist (or whatever else needs to be taken) take their money out, and there is not much left spare. or tell her that the pension people automatically pay the care home first, and only whats left is put into her account, and it's not much.
as her power of attorney, isn't it your job to make sure her care home fees are paid so that she actually has a roof over her head, food on her plate and the care that she needs? so even if she doesn't like it, you are doing what needs to be done to keep her alive with the basics covered.