Kill them in the dead of night as they sleep in their bed and you'll never have the problem again.
Or...you could just deal with it until you move out on your own. Parents are there to question every investment and decision you make at your age.It'll never change, whether its your clothes, hair , friends, whatever. At least they care enough to bug you about it. Whoever said to tell them that at least you're not doing drugs/alcohol probably was right. If you compare the activities that you COULD be involved in, but aren't because of your gym time, they'll probably back off a bit.Don't forget to mention that working out keeps you from having lots of un protected pre-marital sex. too. That should shut them up for good.