Snipers rapidly undermine morale and derail discussions; thus they must be quashed quickly. You can use their desire to be in control against them. Unlike your immediate reaction to ignore the comment, openly challenge their behaviour (not their comments though). Take away their cover by surfacing the sniper--NameTheConflict. Ask them directly if they have noticed any problem. Say you noticed the negativity in their comments and would like to know what sparked such a comment. If they make fun of your facts, ask them if they feel something was wrong with the facts. Often snipers have no basis for their comments, so they more you press them for details, the more their attack will fall apart.
Also use others in the group to norm against the sniper. Snipers try to break social cohesion and thus gain power (MetaWiki:DivideAndConquer). Snipers play off the implicit social rule that one person speaking represents ten, and silence implies agreement. If you demonstrate that the sniper is alone in his or her comments, the sniper will feel isolated and disempowered, and then back away from his or her comments. This is hard to do online without presence technology as you have no idea who is listening, but as long as you can demonstrate that many people agree with you and none (or no one respectable) agrees with him or her you will succeed.
Snipers will use two ploys in defense in order to relevel the field in their favour.
NameTheConflict again. Say that while it may have been intended as a joke, its negative connotations imply disagreement that should be openly surfaced. Force the sniper to either speak their mind clearly or retract the comment.
Snipers tend to test the waters continuously until they are sure there is no room for their weak power play. You must continuously reassert your authority each time. This may be tiresome; other options are to give up on the individual and DissuadeInteraction if you can.
An alternative approach is given on CriticismIsFeedback: transform the comments into FairCriticism and respond accordingly.
FlameWarriors call this the [Jerk]; and when sniping is done in a more purified sense to troll for reactions, the [Evil Clown].
Implied in the above is the idea that SnipingCriticism is UnfairCriticism? or NegativeCriticism?. The target of criticism may receive it as negative or unfair, but that may not be the intent of the critic. If you are the target of criticism and cannot perceive it in any way as FairCriticism or CriticismIsFeedback, and you infer that the intent of the criticism is sniping, do the above or don't feed the trolls.