Assuming no one else comes up with the wiring details, this method should be fine but as with all mains power BE CAREFUL. Also rather than taping a terminal in a warm moist enviroment (loss of sticky) remove the spade lug and place an insulated screw terminal over the wire. Make sure it is unplugged before proceeding and cool ;)
You need a decent multimeter for this. You will find the three elements are wired in "Y" or Star" configuration with one common and 3 actives for each element. The Common goes to the center of the Y and then out to each of the three elements.
Note the positions label and draw a diagram of the wires to the element and remove them all from the element. Digital photo will do also.
Using your multimeter on Ohms (resistance) range at the X1 or maybe X10 settings. Measure the resistance between any 2 terminals and continue around the other ones. You will find that one pair of connections gives a lower reading than the other 2 (half). Of the pair with the lower reading one of them is the common. Using this same method and a process of elimination you should be able to isolate the common and the three actives and disable one of them.
Minor other thought if you only get two pairs with resistance then there is only 2 elements fitted not 3. Pull any one wire in this case to disable one of them for half the power. This is not likely BTW most units seem to run 3 elements.
Hope this is as clear as coffee :)