@Jean Wu ,
Is there admin control to globally turn it on/off by default for all users in that organization?
Sure. As an administrator, you can turn off Teams meeting feature in Outlook client from Microsoft Teams admin center > Meetings: Meeting policies > enter into Global(org-wide default) policy (please make sure that all your users are assigned this policy) > turn off Outlook add-in.
If you want to turn off Teams meeting completely, please turn off the four options as below:
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