Hello,
In Teams, click on the upper right user icon, do you see their company name listed, if so, click on that to switch to it, then go to Settings and to the page in your screenshot, the leave button should now become available for you to toggle off.
If for any reason you cannot get to that account at all, see if you can do this:
go to https://portual.azure.com See if you can sign in.
If you can, click on Manage Entra ID > Manage tenant, click on the company name and then click on leave.