It seems likely to me the zodiac didn't want his ciphers decrypted at all and was just toying with people, he liked the attention, so made his code impossible. Throwing in random cipher characters or purposefully decrypting into junk. These things have been decrypted so many times, with different decryption techniques used for different parts of the letter, sooner or later you're going to wind up with it saying whatever you want. We're at the stage technologically now where the letter could be brute forced, and that hasn't come up with anything either.