Since we have absolutely NO evidence to believe that a shadow can exist separate from its host, we must assume that a banished shadow IS a dead shadow.
Which is why the most you can do is banish it back to the subconscious and temporarily cage it (like Harry did). This is also supported by the rules, where it says you can't just whip up a spell to solve every problem or end run around the story. Magic doesn't work like that. For major problems, the best you can do is temporarily alleviate the problem, or have the spell point you in the right direction (which in this case is convincing the tainted to willingly reject the coin).
And we're not talking about a dimestore mortal spell, here. We're talking a spell significantly more powerful than something that Harry Dresden, Wizard of the White Council of Magic, self-proclaimed as farm more adept with thaumaturgy than evocation, initially deemed impossible.
Steps one through twenty of this ritual are going to be figuring out how it's accomplished.
And if such a spell were possible (and the rules of the game and theme of the novels heavily imply its not), then someone would have thought of it already. And Dresden would have just went to McCoy for a de-taintifying. And the church wouldn't have a problem with people people being corrupted and stealing back the coins.
In addition, exorcism wouldn't work on a shadow, or the Fallen anyway, because they aren't possessing the target, they're there willingly. Unless the target rejects the coin and the source of temptation, he is allowing the shadow to be there (even if he doesn't want it there, he's saying its better than the alternative, which counts as consent).
So, even if you could make a spell to get rid of the fallen/shadow without the target rejecting it, that would be a forcible altering of the subjects mind(the shadow is built of the subjects mind, and if the subject isn't rejecting it, then its there of his own free will) and a breach of the fourth law.
