MMh, i ran into the same problem. My users just don't get it and most of all they want to use the activity wall for real interactions and not so much system bogus information.
Did anybody found a way to keep specific autopostet messages of the activity wall?
@phreak I altered the Activity workflow in my Post Scheduler plugin. It wasn't simple, but what you ask for is doable, to an extent.
It is possible to avoid notifying the Users of such some Activities by setting its Notification preferences to false. In Vanilla 2.0, if both Email and Popup notifications are disabled for a User, such activity doesn't seem to be registered. Vanilla 2.1 goes a bit further, registering the Activity anyway if it's public.
Regarding the display of Activities on the wall, it's possible to alter it by implementing an ActivityModel_ActivityQuery_Handler and modifying the where clauses of the query.
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It's been so long that I don't know what am I talking about. But good job anyway.