Is it possible to prevent a user being in a particular User Group if they are already in another? e.g. A user cannot be a in "Authors" if they're already in "Administrator"? (not necessarily a practical example, but for the question)
As per the current behavior of Joomla user can be in multiple usergroups and there is no hidden or inbuilt functionality available which can be utilized to implement such scenario.
But obviously there are some alternates to everything.
You can develop a plugin which will maintain a mapping of mutually exclusive user groups like User-group A is mutually exclusive to User-group B and so on.
And on Joomla userevents(onUserBeforeSave) you can check the difference between the usergroups and if there is any usergroup update which is not allowed then either return false in the event or redirect or show some error, whatever you like.
There is no feature to enforce Exclusive User Groups in Joomla. Users can be on more than one group.