Two orthogonal issues.
First, an adversary could make a 1 Satoshi input and DOS on the (3) step. You ban that address but adversary has billions more at neglible cost.
I suppose you could set a minimum input amount to avoid this. But still no problem for the adversary, he passes his BTC through a mixer can comes to hit you again and again.
I am sorry to bring you bad news Gregory but with a non-atomic operation you can always be DOS-attacked.  Zerocoin may be the solution?