That is always difficult. In my experience that can be a killer in a small environment in many ways. I suggest dealing with it quickly and honestly. Do you see an opportunity for the person to adapt or is there no hope? Sit down with the person outside the office to have a genuine conversation about your concern and hear them out. It may be appropriate based on the outcome of your discussion to give a certain time period to allow them to implement your feedback, or it may be best to part ways quickly. Either way, always remember that leaders get the behavior they tolerate, good or bad. Nipping this in the bud will help the team members who are a great fit see you value their input and keep them motivated.

Best of luck!