We're doing the same thing that the other moderators have always been doing and have the same privileges in PB&M, but we are moving towards more frequently locking threads in the wrong space rather than deleting them if they aren't disruptive and can still be useful; this gives more clarity to the community about the actions we are taking and hopefully will serve as a tool to show players what kind of discussions are appropriate there.
Yes, we message the player in that thread why it is locked and give them the same kind of information you've come to expect from a deletion notification (what's wrong and where you should post it, or why you shouldn't at all).
When we lock a thread because the discussions inside have gone off topic, it's probably because the topic was inherently unconstructive and going to result in that no matter how many conversations we removed.
Also, we do frequently remove individual comments as well...but then, you don't see those because they are gone.