Generally, there's no need to delete a wiki as other people may want to continue working on it, even if you don't. Other people can adopt the wiki after the founder or other admins have left. If the wiki has a community of people working on it, or the site is thought to potentially gain new editors in the future; then the wiki will usually be kept open. If you see a wiki that someone else started which you just don't like, it's best to ignore it, but if the wiki is being used only for spam or vandalism, please report that to staff using Special:Contact.