wrote in message news:6uIXa.2166
If you can get the seat adjusted to a height you're happy with, you could
weld it in place, or drill a hole through the post, bolt it in place, and
mess up the threads so it couldn't be undone without a hacksaw.

if your bike wasnt stupid light, a rivet or two would look a lot
cleaner than a bolt, and would be more secure. you could always drill
them out later. or for a beater-glue. you could just fill fastener
heads with epoxy. use a punch (to break otu the epoxy) or a
screw-extractor to remove later. or- perhaps even filling the fastener
with something temporary like wax might work. stop a thief inserting
an allen key without a whole lot of fussing about.

thoughts,

anthony