A ibisAccount must be deleted by yourself, please refer to the following.

If you deleted your ibisAccount or SNS account first, your ibisPaint account and artwork will remain in the online gallery.
In this case, we cannot delete your ibisPaint account and artwork, because we cannot confirm that you really had an ibisPaint account. Therefore, please delete your ibisPaint account first by referring to the following page.
Delete Account
Once you have completed the above, delete the ibisAccount by referring to the following
1.Open https://account.ibis.ne.jp in your browser.
2.Tap the “Login” button to log in to your account.
3.Open the “Account Information” tab.
4.Tap the red “Delete Account” button.