People do throw rocks down holes, of that there is no doubt. My impression has always been that a dry stone wall was long ago constructed around Eldon Hole, presumably for safety reasons, and that this was steadily chucked down the Hole over a period of time, being added to as the landowner/tenant/whoever played a losing game of repairing against passers by intent on mindless amusement.
On a much smaller scale, I've seen a similar thing on Mendip. When we cleared out the Longwood Main Sink, much of what we removed was the remains of the drystone wall that Rich Witcombe built around the sink a couple of decades back, at the behest of SWT. After we removed it, the stone was transported a little way down valley & used in the construction of the dam & retaining wall at Longwood Valley Sink. Fortunately, there, it is not a free standing wall as before but incorporated into a bank & thus less likely to be picked up by passers-by & dropped. There's no convenient deep hole or pool to splash stones into, either.