There is a Bechstein grand piano which is in a college chapel. It is free and open for anybody to play. The only restrictions are that it is not played during services or meetings which are never when I play.
However I've found that somebody is being greedy and is disabling the sustain pedal somehow. Some days it works, as if there is nothing wrong, other days it doesn't work.
So when I and others have a go on it, you press down on the pedal and it goes thump as if the hole is blocked for the rod to push.
That is as much as I can discern from a visual inspection.
Yet I can sometimes hear it being played presumably by this person and it clearly is sustaining.
How is this person disabling it?
How do I prevent them from disabling it?
As its not very fair to be hogging it!
Chances are, if its in a public location, its not one person "disabling" the sustain pedal. If the piano is getting knocked around, (or merely "moved" by people who shouldn't touch it) the pedal rod may be getting knocked in and out of place. This is so common that I'm amazed you came to the conclusion someone is doing it on purpose. It may be that this person simply knows how to repair the sustain pedal and you do not.