I wrote "RESERVED FOR VISA" in pencil on all the blank pages, forcing them to use up the smaller spaces left on other pages
I pencilled a neat cross over the blank pages, erasing it as they needed to be used. That worked for a while until some jobsworth, with a face like a bulldog chewing a wasp, at Gatwick told me that it was 'illegal to deface Her Majesty's Government's property'. I suppose her comment was technically valid but her attitude was deplorable. However I ignored her and never had a problem again.