Depends on the persona. Some users join and want to be very active from the very first second. Others will join, check things out and have less interaction. (I will ignore migrating accounts)
Then there are ways of limiting them. Do you want to prevent interaction or delay interaction? You can also make a distinction based upon the way they interact.
Spam can be broadcasted or 1-on-1.
Personally I don’t like shadow moderation because of the doubt it might provide users (I have no ethical reason against it).
What I notice in your question is that you focus a lot on the user experience. And I’m (only now) wondering about which user and in which context.
As for legit accounts, I would check this first, but I think most users will understand if you explain it to them BEFORE they make an attempt.