You can delete your account from the official pixiv app provided by pixiv Inc. using the following procedures.

iOS: My Page > About this app > Leave and delete your account
Android: My Page > About this app > Leave and delete your account

Once your account is deleted, all your data (such as works you posted and bookmarks) will be permanently deleted. This means that data cannot be restored.
It takes about 2 weeks to delete an account, so you won't be able to register a new account with the same e-mail address during that time.