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 DELETE permissions (owner if config is set
'remove_onlyadmins' => 0, and administrators).
Note: Deleting a wiki page is an irreversible operation.
In general, it is recommended to create redirection pages instead of completely deleting an old page, as to not frustrate the visitor with broken links when coming to the site from a search engine. 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.