This is tough. Cremation is about the only accurate method, but, it really does not matter.
You will never get it all out.
The answer is to have mercury going out of the body just a few micrograms faster than it is coming in.
The ultimate goal is to control the balance between what is in storage compared to what is circulating.
We are all exposed to mercury daily in air, food and water. We excrete it in sweat, urine and faeces.
Maintaining the balance in favour of excretion than intake will keep you feeling good, and keep your chemistry looking good.