Before you try to fix this problem, please consider if this is a problem in the first place.
1: find out how much fun the players are having when their characters receive "too much" swag from your game.
2: determine why you run RPGs in the first place.
I think you will find that getting too much treasure is about as unfun as having a PC that can kill a dragon with their bare hands or a magic word. There are some players who don't enjoy this, but I bet there are more players that do.
If you are concerned about being Monty Haul, then just check yourself. How many deadly encounters has their characters gone through before now? Have they or have they not earned the treasure that they receive?
If you are concerned about realism, then realize that there are people in our own world that have made more up to now than we would make if we are doing what we are doing for the next 10,000 years. A singer scrapes along doing gigs for cheap, then becomes a superstar. A business owner struggles along for a few years, then things come together for him and he profits a million dollars. But it doesn't stop. The next year he makes two million. Five years later he makes 10 million. Ten years later he makes 100 million. Once you learn what it takes to make money, the money just keeps coming in.
Please don't use Mordenkinen's Funkiller. Keep that crazy uncle locked up in the celler. Unless the PCs use it first, that is.
You can always give them new things to do with their money. What if they actually defeat the evil overlord? What if they decide to adopt the evil empire and transform it into a funtionally good society? That's going to take a lot of money. Money they just happen to have.
A good character can come to the realization that they could do a lot of good with thier million GP. They could relieve suffering on a wide scale and transform the world to a better place.
Think of ways to positively drain away their resources instead of negativly drain by having them receive new benefits for their loss of cash. Find out what they want in life, then find out how to make them pay for what they want.
Be the DM that rewards success, instead of punishing it.