Guest Access Permission
It is currently very difficult to control Guess Access Permissions in MS Teams. We only have the options to block creation, editing or deletion of Channels, but a Guest can delete all contents of a Channel and even empty the SharePoint recycle bin.
The only workaround I have found to date is the following:
In SharePoint Site Permissions, create a new Permission Level that does not allow file deletion, move the O-365 Group from the Edit Permission Level to the new restricted Permission Level, then manually add Internal Members to the Edit Permission Level.
The above workaround will allow only Internal Members to delete files, so Guests will not be able to delete files.
We do need Microsoft to offer a way, via Teams, to control the access. To achieve this, Internal Members and Guests will need to be put in separate O-365 Groups so that the Internal Members Group gets added to the Edit Permission Level, and the Guests Group gets put in the restricted Permission Level.