Hello Aaron,
I tried your suggestion, but is did not work, so I carried out the following:
• Deleted sub site
• Created new sub site
• Cancelled inherit permissions
• Sent invitation to an external user
• External user accepted the invitation and went to the sub site, unfortunately the external user was still able to gain access to the team site via the icon on the top left hand site that allows the user to navigate up
• I checked the sub site at users and groups and the external user was displayed
• Then checked on the team web site and the external user was again displayed, so cancelling the inherit permissions did not help
• Then deleted the external user from the team site
• Checked on the sub site that the external user had not also been deleted
• External user tried again to gain access to the sub site, however this time an error message was displayed and the external user could not gain access it was denied
In summary, cancelling inherit permissions and inviting an external user to a sub site, means that the external user is also logged at the team site level. The external user can gain access to the sub site and the team site (defiantly not wanted) however deleting the external user from the team site means that he no longer has access to the sub site