Don't act in the heat of the moment
Don't terminate an employee in the heat of the moment, you're setting yourself up for a lawsuit. There is also the possibility the employee might respond in an over emotional way, and come back the next day with a weapon. A good first move would be to suspend the person with pay. Then conduct a thorough investigation, obtain information from all parties involved and terminate the person only if the facts support the charge of willful misconduct or violation of rules. Make sure that "the last straw" is a solid one.
Work with the union
If yours is a union environment, you need to have the union's backing on the termination. You should sit down with your counterpart in the labor union to discuss the problem employee. Hopefully, both of you can agree that you have done all the coaching, counseling, training and disciplining possible. If you don't have the union's backing, the union is just going to file a grievance and get the person's job back.
Being the stressful meeting that it is, you actually may want to rehearse your conversation. Write down your thoughts beforehand to reduce the likelihood of getting tongue-tied, when you are sitting in the room with this employee who has six zillion reasons why you shouldn't terminate him.
Time the event carefully
Be sensitive to timing. Too many companies lay off their employees around Christmas time. You are not creating any goodwill by doing that. An employee was recently fired on "bring your son/daughter to work" day. The boss came in and fired him in front of his son! I recommend terminating an employee privately. When the office is emptying out around lunchtime, you might say, "John, can you come here for a minute? There is something that I wanted to talk to you about. Jay from Human Resources, is going to join us." Always treat the employee with dignity.
Pick a neutral site
I also recommend having the termination meeting in a neutral area, like a conference room. Don't hold the meeting in your office because if the employee gets too upset he/she might not want to leave your office for what seems like hours. I also would not conduct this meeting in the subordinate's office. This is not a developmental meeting, like a performance appraisal interview should be.
Have the direct supervisor terminate the employee
The department manager (not the Human Resource Manager) should be the one announcing "you are being terminated." The HR person's role in this meeting is to provide support, for both the manager doing the terminating and the employee being terminated, making sure the conversation doesn't blow up, and to serve as a witness.
The employee is typically thinking, "Why me, why are you firing me? Joe over here is performing worse than I am! You never liked me!" You'd better have a good answer. You might say, "The last time we discussed this you were suspended for three days. I said that if this happened once more you would be terminated. Since it has happened again, I am afraid that today is your last day." The termination should not be a surprise to the employee.
Never lose your cool
The more hostile you become, the greater the likelihood that the employee will file a lawsuit or a grievance or become violent. Be a little empathetic. I might say something like, "I know this is going to throw your family a real curve ball. But basically, you have done this to yourself. You have violated our safety rules and put your co-workers' safety in jeopardy several times."
Keep it short
The meeting should not be lengthy. Don't begin the meeting with the normal small talk, like "how are the wife and kids?"
Let the employee vent
Give the employee a chance to vent. Using your "active listening" skills will take some of the wind out of their sails.
Explain their benefits
You might continue by saying that, "Jay from Human Resources is going to explain your severance package and what's going to happen with your unused vacation time." This information should also be given to the employee in writing, because they are usually too emotional to listen clearly to those particulars at that point.