Generally, the rule test for this is "can the person do the job they were hired to do", and if they can't, it's fine to fire them, so long as you have properly documented their inability, and given them sufficient time to rectify it (typically 6 months to 1 year). I understand this is unpleasant (and almost feels "wrong") to do, but, ethically, I think that a good manager has an imperative to do this.