If you have an attorney, this is a question best answered by him or her, since he or she is the one most familiar with what is happening in your case. If you do not have an attorney, I would urge you to get one
The general answer is that yes, you can sue. Any claim you might have based upon the actions of the defendant if the events took place in other than cyber-space, you have in cyber -space. Depending upon your state laws and who the parties are, you may have additional claims. Only a local attorney can answer this question for you.
The consequences of suing are and could be severe. I always tell my clients that they should have "clean hands" meaning that there is no reason for the would-be defendant to assert a counter-claim so you don't end up defending a case at the same time. The process will take time, cost you money (most likely) and be frustrating. Suing someone is not a quick fix to any problem. It is always prudent to start with a stern warning from a lawyer that a lawsuit will happen if the measures are not taken to fix the problem and stop further bulling. You should consider speaking with a lawyer before you do anything. Good luck.
This answer does not establish an attorney client relationship and is just my opinion based upon the limited facts presented.
The simple answer is yes, you can sue. However, whether or not you should sue is really the question. It depends on a number of factors. What is the nature of the defamatory statements? How much damage have they caused? How often are these statements published? Can you prove who made them (while you can conduct e-discovery to trace the statements to an IP address, this is a costly process)? Does the defendant have the money to pay your damages, if and when you get a settlement or judgment? These are all very important questions. As far as how serious the consequences are of suing - you should know that litigation is an expensive process, both psychologically and financially. While you may find an attorney to take a case such as this on contingency, it may be difficult, depending on the answers to the above questions. I suggest you talk with a lawyer about possibly sending a cease and desist letter - it could solve the problem before you have to take further action.
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