Sometimes, I just leave them in my wallet and forget about it. If you are being a honest citizen, here's what I would do as a US citizen:

If you worked for those coins from bounty campaigns, you can report the earning amount at the particular time you received them and if they turn out to be worthless, then you can trade them away for loss and write them as capital loss tax and they can be as deductible to reduce your tax bill.

DISCLAIMER: Not a tax advice, please check with your accountant or tax adviser.