stop sharing an activity

[Yioryos Asprobounitis]

I could not find a way to stop sharing an activity even when no peers are connected or even present. 
This was true in os860 and os20, XO-1 and XO-1.5 and activities like write and Turtle blocks.
If this is indeed a feature and not a bug or something I miss, could someone point me to the relevant discussion and/or rational of "once public, always public"?
Thx