Whilst M2 appears to work pretty-much as expected (although I'm still in the build/test stage on 'localhost'), a problem which appears to be two levels deep.
Once someone enters their details for registration, their information is recorded in the database, naturally. If they cancel-out before completing payment, their members list shows the account as pending. This naturally displays the 'Protected content' page if they attempt to access 'members Only' pages - with the exception of BB Press.
Even though the forum is set to members only, there is no protected content notice and all threads can be accessed by all visitors to the site.
In the above scenario where a membership is pending due to non completion of payment, the pending member is then given full access to the forum and is free to create new threads an to comment.
Hopefully I have just missed a setting somewhere - if not, is this a bug with M2.