Anybody can alter wiki contents, and I am wondering "How does Wikipedia prevent spamming?", when even not registered members edit the contents. What you think about it?
Wikipedia has many editors, each editor is usually subscribed to change notifications of the article they created/edited during their Wikipedia activity life span. When you make an edit, one of the editors reviews it and if it's spam he will revert the change to the article previous state.
Wikipedia relies on people to filter out the spam, you can do this too by going to the history tab.
From their About page on abuse:
Handling disputes and abuse
Wikipedia has a rich set of methods to handle most abuses that commonly arise. These methods are well-tested and should be relied upon.
- Intentional vandalism can be reported and corrected by anyone.
- Unresolved disputes between editors, whether based upon behavior, editorial approach, or validity of content, can be addressed through the talk page of an article, through requesting comments from other editors or through Wikipedia's comprehensive dispute resolution process.
- Abuse of user accounts, such as the creation of "Internet sock puppets" or solicitation of friends and other parties to enforce a non-neutral viewpoint or inappropriate consensus within a discussion, or to disrupt other Wikipedia processes in an annoying manner, are addressed through the sock puppet policy.
In addition, brand new users (until they have established themselves a bit) may at the start find that their votes are given less weight by editors in some informal polls, in order to prevent abuse of single-purpose accounts.
While crowdsourcing is definitely part of the way Wikipedia handles spam it's only a very minor part. Wikipedia is using several anti-spam extensions, without these it would be a big spamfest that no army of humans could stop. Around November 2014 these are set up:
- Abuse Filter - Applies automatic heuristics to edits
- AntiSpoof – Blocks the creation of accounts with mixed-script, confusing and similar usernames
- AntiSpoof for CentralAuth – Adds AntiSpoof technology to CentralAuth
- ConfirmEdit - Provides CAPTCHA techniques to protect against spam and password-guessing
- SpamBlacklist – Regex-based anti-spam tool allowing to blacklist URLs in pages and email addresses for registered users
- Title Blacklist - Allows administrators to forbid creation of pages and user accounts per a blacklist and whitelist
- TorBlock - Allows tor exit nodes to be blocked from editing a wiki