Coaches can minimize or prevent it. or tacitly permit it by stoically lookling the other way in games. As you state, Niners let the same players get away with it, time after time; nobody gets benched for a series, etc. I believe Ryans wears his stoic "didn't see anyting" demeanor as a disguise - he's not going to stand there and cheer it.
Dirty D's target the opposition's best players - and the Niners have a history of going after Kupp ,every game we've played them since Ryans took over last year.
Remember Greg Williams? I was ashamed when we had him - he should have been thrown out of the league. Withoutr dirt he couldn't succeed. Not every player was a cheap shot artist or designated head-hunter; just a few.
That's what I see with this Niner D. Not to the extent that Williams coached it, but same stripes, different tiger - the league has driven it underground since Williams-Payton . You can only get away with less, now.
When I see it, consistently, from more than one player on a team, I'm not tolerant and non-judgemental.
To my eye the Niners D has a few persistently dirty players, and
ipso facto a coach who is at the very least complicit with it.
If it ain't you, DeMeco, prove otherwise. Clean up your unit.