what is necessary to fire someone for incompetence?

You don't say whether the accountant is an employee or an outside contractor.  Assuming that the accountant is an employee, the allowable grounds for firing him/her depend on the terms of his/her employment contract (which may not be a written individual agreement, it could be oral, could be found in an employee handbook, or could be a collective bargaining agreement with a union).  If there is no employment agreement, or the agreement does not cover grounds for termination, the employee can be fired for ANY REASON, other than those prohibited by statute (i.e. based on race, gender, religion, age, being a whistleblower, etc.) - the employee does not have to be incompetent to be fired, although it may make a difference when it comes to paying unemployment based on whether the employee was fired "for cause" or not.  However, if what you write about the accountant's work performance is true,  you certainly have grounds to fire them "for cause".