The bonus of writing someone up is that the employee often will tell other employees what a mean and nasty person you are. They are trying to get the other employees support and avoid embarrassment for themselves. Do not discuss one employee's disciplinary action with any other employee. Beyond the when, how and why of discipline remember to step back and assess your hiring practices and communication style. Track your turnover ratio for the last two years.
Excessive turnover is very costly to any business.
Monitor the skill levels of your current employees with fun quizzes and by monitoring their work. Communicate your long and short term objectives and goals to employees. Not communicating your goals and objectives can in itself lead to undue frustration and turmoil. And most importantly, catch your employees doing things right. When they do, give specific praise to encourage the repetition of the positive behavior.