Suicide threats are a hard subject to deal with. In general, there is unlikely to be any real legal action against you if you fail to report it, simply because you can make the case that you didn't know who it was or how to get in touch with the appropriate people. That said, morally, it is probably best to take it seriously and attempt to report it to local authorities and/or refer them to a suicide prevention group.
Even if it appears that they may be joking, it can be hard to tell as suicidal individuals tend to be seeking attention and can come off very similar to trolls. The standard practice, even for trained prevention counselors, is to refer them to a medical professional and in some cases (the later described stage 3, or sometimes even stage 2) immediately to law enforcement as well. It may be a joke, but unless you are a psychologist talking to them directly, you really aren't in a position to make that judgement call. It simply isn't worth risking someone's life on your assumption.
There are multiple stages of suicidalness. It starts with someone who is thinking about killing themselves, but has not yet decided on a way to kill themselves. These people are generally desperate for attention/approval or a way out of their pain. The next stage is when they have reached the point where they have decided on how they would commit suicide but have not yet decided to go through with it. These people are far further along than stage one and have started taking the idea of killing themselves very seriously, but are still trying to see that someone cares about them. Stage three is when the individual is actively ready to commit suicide imminently. If these people are making a threat at all, it is a last desperate plea for help before they end it and it is critical that action be taken as quickly as possible.
In general, it is best to be as supportive as you can be. Reinforce that you value them in the community and don't want to see them get hurt while trying to refer them to a suicide prevention specialist. You can also be working on trying to contact local law enforcement where their IP is, particularly if they are in stage 3. Particularly for stage 3, try to keep them engaged until you can hand them off to someone qualified to deal with the situation.
Possibly join a suicide prevention chat room or site yourself and explain the situation to them so they can help coach you through it and facilitate getting the individual handed off to them (they will also be better equipped to handle law enforcement notification if you can provide them with IP address or e-mail or other relevant details.)
This post is really just a very basic crash course in a much more complicated field that involves quite a bit of training to do well, so getting help to deal with it is critical. If it is stage 1 or 2, it isn't as critical as there is generally time before they would do anything, but it's still probably a good idea to talk to a suicide prevention counselor for assistance.
(Full disclosure: I was a trained peer support counselor in high school and have training on dealing with suicidal individuals and getting them handed off to appropriate authorities and medical personnel. This post is an attempt to summarize some of the most critical bits for someone untrained to go through, but it is by no means exhaustive and I do not have any particular experience in online mediation. (I was phone based in High School.))