IB Plus T&Cs do not appear to make any provision for deleting an account. However, if you ring up and enquire this may be possible. (In the data protection section, it does say that you can "cancel" data).

This, however, is unnecessary. The easiest way to prevent your IB account "interfering" with any other account is to simply stop quoting and using your IB number. Change the FF number in any existing reservations yourself, or by asking an agent at checkin. You are the only person who can associate this number with any current or future bookings - so just stop doing so. Stop logging into your IB account, and stop typing in your IB number.

Even if your IB account is deleted, this will not necessarily help you in this endeavour. The now defunct number will still be acceptable by most systems, and if this old number is still in any booking, your "other" oneworld account won't just automatically receive the miles instead - and neither will you receive the Avios in your Iberia account because you have closed it. Deleting the account won't help - you need to delete the number from any and all bookings.