I have a number of gwent cards that I will never use. They aren't in an active deck; but I can't find a way to toss them into the fire; sell them or otherwise drop them. Is there an in game method for destroying cards you don't want ?

Not that i remember, you just don't use them

Part of the reason is for collection i suppose, using them or not you found them and are added to the collection

Alternatively, you can try with console commands, maybe there is one to remove cards, not sure

Check nexusmod page of the Witcher 3 to see find the console enabler, then look for wiki about commands for the cards

I just found out my list, additem give Geralts thingy, maybe removeitem work, do try