You can delete your account from the official pixiv app provided by pixiv Inc. using the following procedures.
If you have the app installed
iOS: My Page > About this app > Leave and delete your account
Android: My Page > About this app > Leave and delete your account
If you have uninstalled the app
Follow the procedures for leaving pixiv.
In some cases, if you are still using pixiv-related services, you may not be able to leave pixiv. Please go here to learn more about leaving pixiv.
Once your account is deleted, all your data (such as works you posted and bookmarks) will be permanently deleted. Once you delete your account, your data cannot be restored, so please make sure you do not delete your account by mistake. 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.