Merit system producing spam?  I think not.  There is spam everywhere and there will ever be.  People are so creative enough to produce spams without being caught just in order for them to fulfill their bounty requirements.

On another note, I have a theory that the merit system is not reducing the spam users significantly (meaning much less that what we expect).  I just believe that the spam are being transferred from one board to another.  How? just now look at the Meta Board and you will see that people from the other boards before are going here because this board has a higher chance to get merit than any other board.  Hence, this board attract more users, meaning attract more spam posts.