Since a lot users want be able to get cacti here is my suggestion. It would also take money from economy. There could be feature for presidents+ where you can apply for single cactus placement, and that would cost 10K. You can have up to 10 cacti per town and if someone auto farm it would be removed and probably temp ban 3 days+that person wouldn't be able to apply for cactus placement in future.
I would say this would be pretty neat... but raise the price a bit higher. It causes abundant of lag, so this may not even happen.
I just don't see andrew and staff being bothered to check all over towns for placements and auto farming etc on a reg basis on top of giving the server something we know will cause lag. I just can't see the value of it. I would rather have pistons then cactus but knowing the issues they cause I understand why they are not allowed, same with creepers etc.