I don't know how far owncloud and nextcloud have diverged, but in the nextcloud client you can add filters to ignore files by clicking the three dots on the folder in settings.
You can also free up local space by using virtual folders, but it only works properly on windows.
£37k so £2055 per person? I wonder how many weeks that set him back?
There'll be no changes to abuses like this until the punishment for the crime far outweighs the profit. Same with wage theft.