Toss them in near boiling water, couple of rounds of soaking, or perhaps soak in some hydrogen peroxide (the standard stuff that is used for minor cuts and as a gargle).
I don't think I would want to try any stronger/different chemicals, due to the possibility of the caps retaining some of whatever they get soaked in.
Final choice, leave them off and tailpipe -- that's what I'm doing now on most of my cartos, good hit.