Therefore, don't mention the law (pun intended). Even if you're only bringing it up as a hypothetical future possibility, that's still going to be interpreted as a legal threat - a hypothetical future legal threat, but still a legal threat. If you want to talk morality, talk morality, not law.
Sometimes, a community member will mention the law in an ambiguous way - such that maybe they're making a legal threat, and maybe they're not. Such comments are just as corrosive as any other kind of legal threat, so you must neutralise: force the attacker to make explicit whether or not they're making a legal threat. If not, good - adult discussion may continue. If they are making a legal threat, then you can deal with it as such, and in any sane community the attacker will rapidly become a CommunityExile.
But, be aware that vague legal threats, while deeply unpleasant, often mask deep emotional feelings - perhaps feelings that the attacker is unable to express any other way. For example, if your community discourages personal attacks, then someone who feels angry may feel that a LegalSolution is the only way they can vent their rage. So, after neutralising the threat to LimitDamage, try to find the SourcesOfConflict.
There are some cases where it is legitimate to use a LegalThreat.
The latter case exists frequently. For instance, when we tried to change the an innocuous page's copyright to PrimarilyPublicDomain for the MeatballWikiBasicsSnapshot, we ran into one individual who, although agreeing with the snapshot project in general, could not relate to emotional, philosophical, psychological, political, diplomatic, or other means of describing the situation, and instead responded invectively with legalese despite everyone agreeing with his position in principle. He only became more angry when it was explained that we were trying very hard to relate with him, when all he wanted us to do was follow the law to the letter (even to his detriment) and ignore his feelings, which was hard for us following FairProcess to understand. Same SuperordinateGoal, different internal thought processes.
I think this is an important page that will be a mosaic piece to the large topic of "online communication and fighting methods". If we believe that FightingIsBoring, then we must deeply understand all methods of good and bad arguments, of how to provoke and withstand. Maybe even a "trolling tutorial" may make sense to better understand and control all types of unwanted, unconstructive, unproductive online communication. Any community founder needs more than basic knowledge.
By the way: the simplest method to answer a hidden legal threat is to ask explicitely "Do you really want to ..." which forces the attacker either a step forward or a step back and unbalances the attacker. If he hesitates, it's just as bad and will make him look silly. If he answers yes, the chance is still 95% that he won't do it, so he is threatened to lose his credibility. If he steps back, the best he can do is make a joke out if it, but he will look silly anyway. So if you make a hidden legal threat, better make it look entirely credible, so that no-one thinks about asking, because otherwise it's going to backfire inevitably. And even if you succeed in that, it will work only once. So the general advice is: don't mention the law.
What if we can't use the law? We appeal to the community and to fairness. -- HelmutLeitner
There is a side point here, which is that no informal community should endeavour to take truly serious matters into its own hands. (compare the admonition against LifetimeBans)
I remember the case of a prominent U.S. university. The student judiciary board, which ordinarily heard complaints about plagarism, vandalizing school property, petty theft, and the like one day took up a rape allegation. Any outside observer can see that they had no business whatsoever doing so, that it was a matter for the police and the courts. But their tradition of trying to deal with conflict internally without resorting to LegalThreats clouded their judgement.
Where the line exists is a matter for discussion, but there are clearly situations where any sort of community (online or otherwise) is best off referring matters out to the legal system for resolution.
LawsuitParanoia? is a poor answer since anyone who engages with the world in a meaningful way can become a target of some frivolous legal action; it's too important to go on living and interacting to retreat into a lawsuit-proof shell.
Do not use legal threats.
An interesting result of the collision between net.culture and legalists is the "cart00ney" phenomenon. [Cart00ney] is a neologism coined in [news.admin.net-abuse.email] to describe spammers who make LegalThreats to try and get themselves removed from IP BanLists.