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It is not possible to delete user accounts, as all contributions must be assigned to some identifier; either a username or an IP address
All Wikipedia editors have the right to leave Wikipedia for good (ie, permanently). The usual way to leave the Wikipedia project is simply to stop editing. Your contributions remain in Wikipedia. If you wish to resume editing at a later date, you can simply start again by logging into the same account. Old accounts that have any significant edits are almost never deleted or recycled to new users.
If you decide to make a fresh start and do not wish to be connected to a previous account, you can simply discontinue the old account(s) and create a new one that becomes the only account you use. Discontinuing the old account means it will not be used again; it should note on its user page that it is inactive— for example, with the {{retired}} tag —to prevent the switch being seen as an attempt to sock puppet.
Editors seeking privacy per their right to vanish can have their accounts renamed and their user pages and (in some cases) user talk pages deleted.