In the distant past we removed our Office 365 account from the control of a third party. Whilst under their control I was a limited administrator. The third party company continues however to be the global administrator and my status has not altered. How do I gain global / security admin rights so that I may control our site fully and remove these unwanted users?
You have to ask them (the third party) to elevate your account, or create a new Global admin user for you. If this is not possible, you will have to contact support and ask them instead, but you will have to go over a lengthy verification process before they agree to do so.