As the GM's are no doubt aware, people excessively grinding Geffenia (and similar maps that produce large amounts of drops with high NPC sell values) presents a problem for the long term health of the server's economy. The problem is that it allows players to rapidly increase the zeny supply of the server, since the NPC produces the zeny out of thin air, with no corresponding zeny sink. How do you create a zeny sink that is desirable enough to offset 4m+ zeny per hour per player without breaking the game? That's a tough problem, and that's why things are the way they are now. The GM's tried reducing the zeny output of the offending maps, only to cause outrage among the player base, so the changes were reverted, and the long term problem remains.
If it's allowed to continue unabated, the result will be inflation that prices out new players from the gear market. People quit the server all the time for a huge variety of reasons, so for the server to stay healthy long term, new players need to keep coming in. If they get here to discover that they are so far behind established players that they can never hope to catch up, they will get discouraged and quit. However, the people who put in countless hours and millions of zeny to build their farming characters have valid concerns as well. The whole point of the game is to invest time and effort into becoming stronger, and those who have done so are justifiably upset when the fruits of their labor are diminished without compensation.
The solution I propose is this: You know the problem maps, you know the problem items. Reduce the NPC sell price of these items to zero. Instead, create an NPC that allows you to trade these items for useful items that can be sold to other players for a similar amount of zeny. For this to work there needs to be organic demand for the items, and the items need to be consumable. Some examples might be condensed whites, elunium, gemstones, SQI ingredients, or stat foods. This way, the players who put in the time to build efficient farming characters can still earn high amounts of zeny per hour, but since the zeny will be coming from other players instead of NPC's, there will not be an inflationary effect. Increasing the supply of consumables will cause their market values to drop, but there should be a natural floor that greatly exceeds their NPC sell value, since there is organic demand for the consumables. After a few tries, the GM's should be able to tune the zeny/hour of these maps to a level that is satisfying to the players that farm them, while making consumables more affordable for newer players.