Existing wiki pages can be completely deleted using the Delete Page or the Delete this Version buttons at the bottom of the wiki page. These buttons are only visible for users with WIKI_DELETE permissions.
Note: Deleting a wiki page is an irreversible operation.
If you want to delete a page because you actually re-created a new page with the same content but a different name, it is recommended to keep the page and use it as a redirection page instead of completely deleting it, as to not frustrate the visitor with broken links when coming to the site from a search engine.
In this situation, chances are that you actually wanted to rename the page instead of doing a copy + delete. The Rename operation also offers you the possibility to create a redirection page. A redirection page is a short page that contains a link such as ?See SomeOtherPage?.
However, deleting specific versions or even complete pages can make sense to remove spam or other abusive submissions.
See also: TracWiki, TracPermissions