If I understand your problem correctly, it sounds like the fundamental issue is that you are trying to block your former company from accessing your former company account because you used it for setting up accounts on other sites. Depending on your jurisdiction, this might even be considered something your former company could go after you for since you are trying to prevent them from accessing their resource which you misused (even if the misuse was unintentional). It is unlikely that you had ownership of the account, so you are effectively trying to prevent your former employer from using their property.
Your best bet is probably to talk to those sites that you used it on and see if you can switch those accounts that are your personal accounts to be based off a personal account that you control. If they are also business related accounts, then you likely no longer have a right to the accounts and shouldn't be worried about it. If you are worried about your company using your name after you left, you could possibly ask the sites about closing the account for you.