Hi,
I read about a very good idea elsewhere but can't remember where.
If there is some form of step inside the seattube (on my bike the
end of the plastic insert), then drilling a small hole and a tiny self
tapping screw can serve two purposes. It can act as a stop for the
your preferred highest seat height and prevent theft, probably a
opportunist thief wouldn't notice the screw and wouldn't be able
to work out why the seatpost won't come out.
If this is what you meant by a screw all along, then I didn't twig.
rgds, sreten.