There is no way to actually remove them, your only option is to apply different dyes instead. (You also wont get the dye stamp back, I'm not sure if you're aware of that or not.)
Dye stamps are a really bad design IMO. They should have added a system like the race change and appearance change tokens (but cheaper) - non subscribers can apply dyes they have unlocked to a costume, or remove them, just like subscribers can but they have to buy a token from the crown store to save the changes.
They would actually have gotten a lot more money from me if they'd done it that way. When costume dying was first announced I assumed it'd work similar to that and I was planning to buy dye stamps for all my costumes. Instead I bought a 30 day subscription, which I realised later was actually cheaper, cancelled it immediately so it wouldn't renew and dyed all of my costumes for £9. (Plus I got the crowns, more expensive if you look at the price per-crown than my usual 5,500 packs, but not too bad.)
PC EU player | She/her/hers | PAWS (Positively Against Wrip-off Stuff) - Say No to Crown Crates!
"Remember in this game we call life that no one said it's fair"