While we can't switch on and off just one adjustment brush "pin", we can dial it right down in power, so that it essentially shows no effect on the image. Then later, dial it back up again to its former strength.
LR keeps track of the adjustment(s) involved and if several have been used, of their relative proportions, and whether positive or negative.
As of LR 6, it is now necessary to hold down the Alt (Option) key while dragging the cursor sideways on the pin (so as not to just move the whole adjustment around).
LR5 and earlier, did not need this key to be pressed.