You've put your finger on the problem with some progressive discipline policies -- employees sometimes have a way of coming up with misconduct that far exceeds anything you might have imagined when you wrote the policy.

Technically, your options depend on how your policy is written. Did you reserve the right to fire for serious offenses? Does the policy say that you have the discretion to decide how to handle particular situations, and that you might depart from the prescribed steps in some cases?

Unless you have led employees to believe that the disciplinary steps will be followed in each and every case, you probably have enough wiggle room to fire the thief. This is particularly true if you are an at-will employer -- one who has reserved your right to fire employees for any reason and has not offered them job security.

If your policy appears to tie your hands, it's probably a good idea to consult with a lawyer about your next steps -- both in dealing with your sticky-fingered bookkeeper and in revising your policies to give you more leeway in the future.