I don't know what's still out there, but there used to be a version of the "Fox Police Lock" that consisted of a brace bar, about as stout as an XJ track bar, that set into a socket in the floor, and at the top was attached to a lockable loop, usually right in the middle of the door. When unlocked, the bar would slide up through the loop, allowing the door to open, but when locked, the only way to get through was to demolish the door. Pretty ugly, though.
I googled "Fox Police Lock," but what came up was a different sort, so I don't know what's available. I think it was also known generically as a "tenement lock," but I haven't searched hard for it.
If you don't need to unlock it from outside, how about just getting a good big slide bolt, perhaps the sort made for gates or barn doors, and mount it on the door facing down. Drill a hole in the floor.