If you find one of those cheap erasers that fit over a pencil that has its own eraser bitten off, or worn down, you can put that over the nipple to catch your hammer blows. Also, those stick erasers, that are about 3 inches long, and an inch wide, and half an inch thick, pink, that you still see in office supply stores, and stationary stores, can be cut up to make a number of plugs to put, one at a time, as they are used, over the nipples, to catch the blow of the hammer.
I use the very small O-rings that fit over the stem of the nipple. One or two of them are tall enough to catch the skirt of the hammer, and keep the hammer from pounding the top of the bare nipple. The 0-rings cost now about $.02 each, but you usually can get a " deal " if you buy a dozen, or more at a time. Either way, we are talking pocket change to solve this problem.