Spammers like to use Wikis to improve their Google-Rank by posting their URLs to the wiki pages. DokuWiki utilizes different methods to avoid this.

As an active method against known spammers their URLs can be blocked. So if you try to post a known Spammer-URL your save will be aborted. See blacklist for details how this works.

To discourage spammers DokuWiki uses a feature called DelayedIndexing. In general it tells searchengines like Google not to index the pages until they are older than a specified time. In general Spam is removed relativly fast from Wikis, if a page has not be changed for some time you can assume it does not contain any spam so indexing is allowed then.

Additionally you can make use of the several (anti)spam plugins, most notable the captcha plugin.