SO THIS IS MY PLAN. I reckon I should now stick a A4-sized
"Please Do Not Park Here" notice on the windscreen of these
regular parkers and make sure the notice is attached with some
sort of glue that takes them a long time to remove.

I would suggest you do drivers door window,
they might not be able to drive their car away otherwise.

Someone once wrote on a friend of mines window with lipstick,
as he'd left it in our underpass, not actually in the way,
but they were obviously having a bad day.
It took him a fair bit of work getting the window clean,
that stuff is really greasy.