Here is a little trick I learned while doing a bit of troubleshooting on older Tesoros and Bounty Hunters, etc. After I discovered it many of the old timers started doing it to their own detectors:
When hunting in discriminate OR all-metal, you can duck tape a steel washer the size of a dime to the bottom (or top) of your coil. This reduces it's sensitivity period, and to iron too. It lowers the set point
of the parameters of the automatic gound balance null. This overdrives the auto return to threshold and also makes the detector not see as much iron because it is working overtime to cancel the little piece of iron you already put into it's detection area.
My experiments here at home have shown that if you try just a small piece of steel first (about the size of a half-dime or smaller) and slowly try a piece of steel a bit bigger and bigger, little by bit as in up to the size of a half dollar, you will find that eventually the detector will be overdriven so much that it just about chokes the thing down to only seeing 3-4" in the air! In fact, it will even stop working at all if you tape too big a piece of steel on the coil.
This does not hurt your detector to use something the size of a dime because there is not enough causation of resistence in the circuitry to heat anything up doing this anyway. After all, in very trashy areas you get it into that mode off and on anyway, but not constantly, although detectors with extremely fast retunes are affected by this trick less than others.. Trying it is easy, it doesn't hurt the detector, and the tape comes off real easily. The key is, that if it is constant, it is not much different than if you had a special extra "iron mask" switch on your detector running all that time. If your detector is too slow to recover it may not work as well, and it may not work at all.