Normally, an article of clothing that never deteriorates and doesn't burn is a good thing. However, these masks can still become tainted. You can't burn em, so what's a warcrimer to do?

A. Leave the tainted masks on the ground forbidden, preferably outside. Eventually you'll find a trader that will buy it off you, albeit for a huge discount.

B. If the body wearing the mask is destroyed, all clothing goes with it (including the mask). You can set fire to it, cremate it at a crematorium, or let it rot away to nothing: all of these will remove the mask as well. (Note that butchering the corpse will not work: the mask will be dropped along with any other clothing)

C. Form a caravan of one person, have them pick up any tainted masks. Send them to an adjacent tile. As soon as they leave your map, click on the caravans inventory and abandon the tainted headgear. Then you can right click and instantly re-enter the map.

D. Drop pods: the next time you're sending goodies to a neighbor via drop pods, throw a tainted mask or 2 in there. They won't object and you'll be rid of it.

E. Burial: If your colonist got shot to death wearing an excellent clown mask, maybe they deserve to be buried in it too.