A good place to start is with your policy and procedure. The more you can attach a negative behavior to a policy violation, the stronger your positon becomes. Most agencies have some type of policy that covers conformance to rules and laws, treating fellow employees with respect, and some type of unsatisfactory performance clause. Most unwanted behavior will fall under one of those broad categories.
Once you get your documentation together you will have to meet with the offending subordinate. You have four goals to accomplish: you need to communicate your concerns, determine the cause of the negative behavior, identify avenues for improvement, and ultimately improve the employee's performance.
When you communicate your concerns, avoid the uncomfortable small talk and just get right to it. You must be direct and candid by speaking in specifics. Make sure you give examples of what is unacceptable behavior and what you deem acceptable (to compare and contrast). Explain what it is exactly you expect out of the employee.
Next, try to identify the root of the problem. Is it home or work related? Is it something you can help with or do you need to refer it to someone else? Does the employee have a justifiable grievance? Does it involve a training issue? Regardless of what the root cause is, the person's poor behavior can't continue. You need to make that very clear as you offer to assist the employee in dealing with any issues he or she may have.
After you discuss the whys of it, you need to focus on the "how to fix it" part. You need to reach an understanding on the corrective action, set up a follow-up date to check on the progress, and continue to monitor for improvement. It's at this point you must ensure that your employee understands the need for your directness but it's also up to you to leave him feeling you care about his opinions, concerns, and that you are there to help him. It sounds like a contradiction but it's not. You can do your job as a supervisor by holding them accountable and still show concern for your subordinate's welfare at the same time; to be truly effective you need both.