Can you point to some other times in history where the threat of being beaten up has been effective in eradicating an ideology?
I can point to plenty of instances where driving an ideology underground only fuels its growth...
And look at the simple logic of it- if the guy in the big KKK hood says 'the establishment doesn't want you to acknowledge this' and he then gets beaten up by mainstream majority people, you've just proved him right in the eyes of a would-be follower.
It's like if you found someone who knows nothing about astronomy, and told them 'today the sun will rise at 7:15am and set at 5:43pm, and the moon is mostly made of bleu cheese' you've predicted two things correctly so that gives you credibility when they consider the 3rd.
If I said that to you, you'd say 'you looked that up on Google, anyone can do that, and it's well known the moon is made of rock'. But you are knowledgeable about astronomy (on a basic level at least).
This works with the KKK person because chances are the KKK person has had limited or no actual contact and understanding with black people. So he sees news reports of inner city black people doing crimes and it becomes easy to convince him black people are somehow inferior. And 'THEY don't want you to know the truth' is a powerful message for someone already interested in counterculture / dislike of the mainstream.
That's why Daryl Davis is effective- he sits down with the racist, who has a mental image of what a 'black person' is, and he's not that. It's like putting you on a rocket and flying you out to the moon and saying 'okay we're here, where's the cheese?'
And that's why violence ISN'T effective- because the racist is expecting violence, so being violent only reinforces their belief.
In that sense you're right- some people only understand and respect force, and the only way to stop them is to use force against them. The analogy of Hitler might be compared to hardened criminals- being nice to a criminal won't stop them from victimizing you.
This is a different battle though. The small % of racists who want to burn minorities' houses down, they must be met with force.
Force doesn't win hearts and minds though. Force is intertwined with fear- 'if you do xxx, force will be used against you'. So it might stop some church burnings, but it doesn't stop racism. Force doesn't win 'hearts and minds'. Force doesn't convince a racist that they were wrong. It might make them too afraid to speak up, but it doesn't win them over, and they will only take their message underground, where it will thrive.