[btcfn]

Impossible. You'd have to somehow convince ALL miners to not process them.

Fungibility prevents you from simply blacklisting these coins.

http://themonetaryfuture.blogspot.it/2013/12/why-bitcoin-fun...

[vladd]

Actually you just need to update the code and push it via the update mechanism.

[btcfn]

Code of what? Bitcoin is not one central code base with a global update system.