To answer your first question: It's pretty easy to track where bitcoins go. People have tools that visualize this (I've seen them). I don't know of one off the top of my head I can point you to, though.
To answer your second question: This idea has been discussed at length on bitcointalk.org by a lot of people. The idea is known as "tainting." There are a lot of problems with making it work in practice, so the concensus seems to be "don't go there."
To give you a sample of the (IMHO valid) objections: For one thing, it's difficult to verify which coins really should be tainted. (How do you know someone claiming certain coins should be tainted is trustworthy?) For another thing, tainting creates two "competing" bitcoin currencies, black coins and white coins. Finally, there is no central authority, so it would be challenging (though perhaps not impossible) to come to a "general concensus" on who should be in charge of declaring coins tainted.
To answer your second question: This idea has been discussed at length on bitcointalk.org by a lot of people. The idea is known as "tainting." There are a lot of problems with making it work in practice, so the concensus seems to be "don't go there."
To give you a sample of the (IMHO valid) objections: For one thing, it's difficult to verify which coins really should be tainted. (How do you know someone claiming certain coins should be tainted is trustworthy?) For another thing, tainting creates two "competing" bitcoin currencies, black coins and white coins. Finally, there is no central authority, so it would be challenging (though perhaps not impossible) to come to a "general concensus" on who should be in charge of declaring coins tainted.