The problem would be that such refusal may itself be immoral.

As with all things the devil is in the details: Who is going to decide to issue the directive that "bitcoins ABC" are not-spendable? What is the process of appeal? (sounds suspiciously like you are advocating a central authority to control how BTC are spent).

Moreover, simple laundering would render enough plausible deniability to defeat the purpose.

Indeed. But it might work for MyBitcoin, MtGox etc to get together and agree to ostracise any coins that have been stolen. This might be perceived as a useful service by customers of those sites.

As freetx says, the devil is in the details.

It is pointless and to me it sounds like marking notes by the government or whoever. Just like cash, Bitcoins will be used for the good and the bad, and that's natural. If I pay an assassin to kill someone, that cash doesn't get "polluted" and at some stage it could even be donated to some charity.

Of course I don't condone scams or murders, but I don't think banning some bitcoins from circulation is a good idea.

If I rob a bank and a dye pack goes off, that cash does get polluted... just something to reconcile.

I am willing to buy your immoral BTC at a discount. Say.... 5% below market price?

Let the bidding commence!

It's not "pointless". Trying to ostracize criminals is important. But this idea in particular is not practical for the described here:

I don't if that's what you meant, but I think that if bitcoins adoption grows faster than governments reaction capacity (like the Internet did), I think they'd eventually create something like a "whitelist" of addresses, which they would control as they do with bank accounts - no anonymity, taxes etc.
No business would be authorized to use an undeclared address, and transfers from declared addresses to undeclared ones would be illegal. Foreigners would be given a way to convert their undeclared addresses to declared ones, probably by paying fees.

My point was that the courts might then reject your plausible deniability defense, "How could I have known those funds he paid me with were gotten illegally?"

Also, if you have a hard time spending those "dirty" bitcoins you keep on receiving, or they have a lower market value than "clean" ones, then you'd probably feel pretty compelled to run some patch that recognized "dirty" or "clean" bitcoins, whether its blacklists they're using, or whitelists.  The bigger businesses would fall in line for the most part since they're easiest to police and they've got the most to lose, and their customers would in turn.  And their customers' customers, etc.  And this would surely get rolled out incrementally somehow so it would feel less invasive and burdensome.

Sounds pretty plausible to me.  And it makes me wonder what could be in store for the homogeneity of bitcoins if lots of different governments have their own separate lists of "dirty" or "clean" bitcoins.