Medic, the elderly man you speak of, I assume the shiners he was throwing out, were caught from the pond in question? I'm not trying to be dense, just trying to clarify.
This leads me to think of something else to consider. How many of the small brim are being thrown out also? If larger Bass are in fact being harvested, that could definitely lead to the large shiner population. With a larger population of big shiners present, you can bet they are spawning very well. With a reduction of your brim population, there is even less of the shiner spawn being eaten by the brim, naturally,
resulting in the explosion of Shiners.

I've also came to understand that Shiners are great robbers of nests. If the shiners are too abundant, they could be wrecking the spawning success of the Brim.

I am by no means an expert, just a layman who has gained a bit of understanding from this site.

I hear all the time, from people I know who claim to be experts. They know how to catch fish, so they automatically understand the dynamics of a fishery. I hear many talk about while fishing public and private waters, if they catch what they consider to be an undesirable fish, to the bank it goes.

One guy told me, he doesnt like to eat bluegill, so he throws everyone he catches, out onto the bank, regardless of size.But yet he searches for large Bass.

I'd go ahead and put up a couple of signs. Even though some may shoot them full of holes, at least they will be announcing their presence with the sounds of gunshots. Put the word out, that the days of free-for-all are gone. I'm not going to go off on one of my rants here. I think Bob Lusk put it best on another thread. "People think Pecans, Watermelons and Fish are free".

Give a man a fish, and he will eat for a day. Teach a man to fish and he will sit in a boat and drink beer.