If coins are tainted by virtue of being used by a client which performs privacy related operations, and if retailers refuse them (because, say, there is a government mandate to honor the tainting authority's output), then I'm using the client which does not.
I can imagine a Coin Tainting Authority (CTA) in some dystopian future--initially implemented to blacklist the bounty of some great crime--that slowly grows more and more prejudiced. I can
see how such a system could arise.
But how would this tainting authority discriminate coinjoin transactions of untainted coins where the inputs are controlled by a single entity, from coinjoin transactions where the inputs have multiple owners? (see:
https://bitcointalksearch.org/topic/coinjoin-bitcoin-privacy-for-the-real-world-279249) Stopping this
legitimate need for privacy is a more difficult proposition--both technically and politically---than the CTA declaring the coins at 1IStole1BillionDollars398dj as black-listed.
In any case, I expect any credible "tainting" efforts to be several years away, if they happen at all
(I personally think blacklisting/tainting will not work for technical reasons). And, in the absence of a truly one-world totalitarian government, there will always be some
untaint-for-a-fee mechanism: for were not control and profits from money-laundering what motivated our hypothetical CTA to over-reach in the first place?
