Hi Maurice,

Thanks for your question, I hope I can help with this. What you'll need to do is:

Go to '[course name] > Course administration > Permissions' and choose to override the Authenticated User role. Perform a search for "resource" and set 'mod/resource:view' to 'Prohibit'.

By changing this setting you should find that the "Resource" activity will be hidden from the view of an Authenticated user (a registered user but not enrolled on the specific course). You can also apply the same changes for a guest (where guest access is enabled.)

I hope this provides the information you require to reach your desired outcome. If not, please let me know and i'll be happy to advise further.
Thanks

Tom