It's important to distinguish trolling from constructive criticism, or even outright criticism. Trolling usually provokes people by saying something philosophically negative about the current topic, while criticism should point out a fairly specific procedural or functional problem. Whereas a troll might say, “This whole company stinks and makes a crappy product. You should go check out my company instead.” a critic might say, “The export to PDF feature in this software crashes if the file is more than 10 pages long” or “The glue supplied with the build-it-yourself table is weak and doesn't bond.”
Keep in mind that trolling is a rare occurrence in wiki sites where a physical community parallels the online one, i.e. in a wiki used for collaboration within a company.
One place to watch for trolling is a public support or documentation wiki, where someone who doesn't like your product (usually not a customer) might post overly negative, disruptive comments. One way to reduce the chance of this happening is to require registration to edit the wiki, though this may deter some otherwise legitimate users who just don't want to complete a registration. If you decide to require registration, don't distract users with unnecessary steps - make sure registration requires only one step and as few clicks as possible so users can get right to posting.
When a WikiTroll disrupts progress, if possible, meet with the person and find out why they're trolling. Sometimes what seems like wiki trolling is the result of a person not understanding wiki community etiquette. If a meeting is not practical or possible, email is another good option. If no email is available, most wiki tools have a topic:talk or a user:talk page for discussions like this.