What if an alternate currency had a large reserve of coins which would be used for a giant coin mixing service for that currency ?
For example opencoin and their 80 billion spare coins could accept "tainted" ripples and swap them out for fresh coins.
Is there a way to do it in a decentralized manner?
If you are going to count on a central organization to remove taint, any laundry would work as well. Am I missing something?
Best way to do this is using blind signatures and predefined units. OpenTransactions should be able to do this, but it's not too hard to implement anyway. You need a client-side program though, which could be javascript like ripple.com client.