Not entirely true, as that unspent output need never be touched. I can move my other coins which I had in this address to another which is now clean, without touching the dirty dust that you sent me.
It's not necessarily dust. Also, I thought we were marking addresses and not individual outputs.
There's no way to "mark" an address on the block chain, other than by sending it coin, that is, creating an unspent output to that address. The owner of the address however never has to touch that output.
In some sense the address is marked, but the owner hasn't spent the "dirty" money there and doesn't have to. He can just leave it and never touch it if he/she so chooses, and still have normal access to the rest of his/her funds.