The next easiest option would be to "write over" the seed phrase, so to speak - stamp every letter of the alphabet on top of every character already stamped on each washer. They will quickly become unreadable and therefore unrecoverable.
That's what I did when I had messed up one washer.
I didn't want to even leak a single word of my seed to anyone going through my trash
(nobody), so I just hammered multiple random letters on top of the ones I had already stamped.