The only way is to destroy the card, bring up your deck and treasure card section, next find the card you wish to be rid of and right click on it, a confirmation window will pop up verifying that you wish to destroy the offending card, confirm and the card will be gone.
Okay, open up your spell book and click on treasure cards. Then go to the Polymorph Ninja pig treasure card and right click your mouse. It will ask you if you want to discard it, click yes. Keep doing it until all of them are deleted.
That, or start using them, those are currently the only ways to get rid of them.
I wish we could sell them to the librarian to at least get some amount of gold from them. I never use the polymorphs and I really dislike trashing something especially when I've gained it from gardening, battles or purchased packs. Or feed them to the pets. I do believe there should be a level limit on them to prevent them from being used in pvp...although I don't do the pvp thing and don't really have any wish to.
linny913 makes a good point, we can sell nearly everything including crown gear, but can't sell TC that we may not want, use, or can't trade. I vote for selling of TC as stated at TC vendor if they can't be auctioned or traded.