First and foremost, it is never a good idea to overlook serious flaws just because an employee is new. Yes, you can cut a new employee a little slack for the short-term, but it will become necessary to address problems in an efficient manner if it seems the employee is showing a complete disregard for the job. Sadly, some employees approach their position with a sense of entitlement. Establishing proper rules of behavior is critical with employees of this sort.
When you do discuss the matter with the employee, be as detailed as possible. Present documentation of the problem. This will cut back on the employee's ability to try and deny or diffuse the criticism. Remember, you do not want to debate the employee's conduct. You will want to present facts about it.
Of course, a little preventive maintenance goes a long way as well. One major reason why employees do not work out is because their job duties, descriptions, and expectations have not been clearly laid out to them. Consequently, they perform in any way they deem fit because they establish their own rules of behavior in the absence of clear clarification of rules of order. So, lay the foundation of acceptable behavior from the outset. This could reduce potential problems to a great degree.
When employees are not correcting their behavior after being told about it, it is best to issue a written warning. This will clearly put the employee on notice to cease disruptive behavior immediately. It is also important to document the employee's additional disruptive issues in case the employee attempts to challenge any adverse actions. However, a written warning is usually enough to make the employee fly straight.
When worst come to worst, be ready to terminate when the time comes. Some managers may have severe reservations about firing a problematic employee. But, it is important to look at the logistics of the situation when such sentiment may rear its head. In some instances, it is vital to fire an employee for the health and productivity of the office as a whole. All managers need to understand this since they all have a duty and an obligation to make sure their employees are productive in the work environment.