About 500ml of blood is removed (a procedure called "venesection") initially once a week to reduce iron levels by stimulating the body to make more red blood cells (which contain iron). Depending on the amount of iron in the body, the initial treatment may take one or two years. Once the excess iron has been removed, venesections are done about four times a year to prevent iron building up.
In other words, it is one of those rare medical conditions where our best treatment is bleeding. Interesting.