If the ISA is with Barclays then then Barclays should stop you depositing more than what is allowed by law.
I have a friend who's mother deposited the maximum amount in a cash isa and went down with dementia in the October I think.
He thought he knew what money his mother had and went and opened a cash isa for her in the February when the LPA came into force not realising the mother had put the maximum in the previous April with a different bank.
He just got a warning letter from the Inland Revenue but was allowed to keep both ISA'S.
The letter did however say if it happened again all ISA money would be taxed that year.