Asses the Situation
Before you act on impulse, sit back and think about exactly what is being said. Is it legitimately slander? Is the person trying to harm your reputation? Are they making irresponsible statements out of jealousy or anger? If so, then you need to take action, but give yourself awhile to cool off before you do anything irrational. The last thing you want to do is try to defend yourself against the individual and make the situation worse. Walk away for an hour, try not to think about what's going on, and if possible, talk the situation through with a friend or family member. When the adrenaline rush has subsided and you are thinking with a clear head, that's the time to take action.
Record the Slanderous Statements Immediately
When someone suspects they're on the verge of trouble, the first thing they're going to do is try to hide the evidence. That's why you need to make a record of it as soon as possible. You can save the pages that contain the slander as HTML documents by navigating to the page, right-clicking anywhere on the page (besides on an image) and choosing "View Source," then saving the document to your desktop with the extension .html. You can also take screencaps of the statements by using the "Print Screen" feature on your computer, or using Firefox's Screengrab plug-in.
You can also request the website that hosted to statements to provide hard copies of the webpages that contain the statements if they are deleted later on, and you decide to initiate a lawsuit against the person. Nothing deleted from the internet is ever permanently deleted, and it's very easy to pull up past documents showing the statements were made and who made them, including the IP address of the person, to prove that they did it fact spread the slander.
Compile a List of Statements
Once you've made copies of the statements, grab a pen and paper, or open up a new word processing document. Then, review everything that's being said about you, and point by point, right it all down. Separate statements into what's true, what's definitely not true, and what's been said based on speculation. Afterwards, go back and separate the outright lies and the speculation, and listed them in chronological order.
Go through your list and think of how you can definitively prove the statements aren't true. The way to prove something is slanderous is by offering the absolute truth, and if the statements are fictional, you should be able to show this very easily.
Draft a Letter to the Individual
Now that you've gotten concrete evidence and a list of slanderous statements, it's time to stop the slander for good. If you have an attorney, enlist them to write the letter for you; not only will they know the appropriate legalese to use, they can make "threats" of legal action against the individual if they don't stop the slander, and you have a much better chance of stopping slander that way. If you don't have an attorney to help you, you can write the letter yourself.
Start by addressing the person by name, if you know it, or use "To Whom It May Concern" if you don't. Open your letter by stating you have been made aware of some statements the person has made online, and that you know the statements were made despite the fact the individual knows they weren't true. Politely request the person remove the statements immediately, and issue another statement saying that the statements that were previously made were false, and that they apologize for their actions. End your letter by letting the person know you will be taking the matter to court if they don't do this within a specified time. If the statements are particularly damaging or personal in nature, go ahead and give them a 24-hour time limit; this is more than enough for an active user to respond and take the appropriate actions.
Let Administrators Know What's Going On
You should copy the website administrators on the letter so they know action has been initiated, and so they can help you enforce the request. You may also want to send them an additional message, letting them know the circumstances, if you haven't already. Be sure to let the administrators know that you will not be holding them liable for the statements (unless you plan on doing so later) and they will be much more interested in helping you get the statements off the website and punishing the user appropriately.
Follow Up and Take Action
If the person ignores your requests, or flat out refuses to remove the statements, it's time to take things to the next level. Keep a record of every single e-mail the person sends you, and any responses you send. Also keep track of the person's posts, to see if they make any additional slanderous remarks. If they post any more slander after you warn them, they'll be likely to receive a much harsher punishment if you win your case at court.
Don't fall flat on your threat to initiate litigation. Suing someone for slander, even without an attorney, is certainly something you are capable of doing. Check out my other articles on how to sue someone for slander, and file a complaint the very next day after the person ignores or refutes your request. If they beg you to stop the lawsuit, it's at your discretion whether or not you want to let them go. But if you want to ensure that no one - especially the person making the slanderous statements - will even think about trying to soil your good name if you show them you are serious about stopping slander as soon as it starts.