Hello Daniel,
I can think of 2 possible options if the charcoal doesn't work.
1. Place the headphones in a bag of rice.
2. Worst case, is to place the headphones into a container of isopropyl alcohol. Let sit in that for about 1/2 hour, remove and air dry it will kill any bacteria causing the smell. Make sure the headphones are completely dry before using. The alcohol will dry pretty quickly, you just don't want to damage the speakers while they are wet.
Good Luck!
Rich lilja