Now, does anyone have any idea how to dispose of these washers?
I missed this question yesterday, but I'll respond anyway because it's fun 
I'd say you can give the washers the same treatment as a hard drive: clamp it in a vice, get an angle grinder, and destroy the letters.
I could toss a washer: down the toilet at home
Don't do that, it'll just get stuck in the pipes very close to your toilet.it would drive me nuts knowing that the washers are not in my household anymore but sitting outside where anyone could pick them up with my seed still stamped onto them.
It becomes interesting when thousands of people are disposing of their washers. Even if you'd find them all, you still wouldn't be able to use them.Still, this is wholly unnecessary. Just stamp over them, make them unreadable, and throw them in your trash.
Even better if you use different trash bins: one per week at home, one in the office. Hide it in something dirty like coffee grounds or diapers.