My point was ANY scheme you come up with will eventually come down to "Does this transaction input have a high enough percentage of 'badness' for me to say no, I won't take it."
Whatever percentage you choose, the bad guys will very likely figure out ways to make their transactions just barely pass your purity test.
Which is why I mostly think starting down that road is probably a bad idea.
On the other hand... "security theatre" can be good public relations. Make the bad guys jump through two hoops and then feel good about how tough you are on crime....