I'd say yes (until the 24 hours run out and the pebble stops being time-stretched).

Similarly, if you go back 6 hours in time and, say, cast a spell that creates fireworks visible from a large distance, you've just prevented everyone within that radius from using a Time-Turner.

Finally, I'm mostly sure that the incredibly-small-scale changes caused by simply going back in time 6 hours propagate quickly enough that only one person on Earth can use a Time-Turner on any given day.

So yeah, there's a bit of a problem here. Presumably this is one of those "it works how Merlin would have expected it to work" things.