Remove User Permissions: You have chosen to delete too many groups at once. Select fewer groups and try again.

If you run into the issue of removing many individual users (particularly if you're using item level security this can happen quickly), you can only delete a maximum of 500 users at a time.

If you have more than that, you basically have two options:

1)  Click each checkbox individually and remove in groups until you get under 500.
2)  Open up Chrome, press F12, run the following script in the console:

var i = 0;
$('input:checkbox').each(function () {  
   $(this).attr('checked', true);
   i++;
 
   if (i >= 475)
   {
 return false;
   }
  });

This will select the first 475 users, and from there you can remove the user permissions.