there is no such thing as an individual "bitcoin". It's just a numerical balance associated with a wallet. There is no way to mark a certain subset of bitcoins as invalid.

At best you could flag wallets, and any wallets who've made transactions with those wallets, etc., etc. which is basically what the people on reddit are doing to "trace" the coins. But, as many have mentioned before, it is not a very accurate way to trace the transactions especially if a service like a tumbler is used.