This is not a productive line of questioning, because currently in America the police don't stop armed neo-nazi rioters. Instead they do nothing about or actively aid them.

https://www.usatoday.com/story/news/factcheck/2020/08/29/fac...

https://www.theguardian.com/world/2018/feb/09/california-pol...

https://slate.com/news-and-politics/2019/06/armed-neo-nazis-...

https://www.washingtonpost.com/nation/2020/08/22/portland-po...

https://www.insider.com/police-salem-oregon-protesters-stay-...

https://www.esquire.com/news-politics/politics/a33798854/jac...

In Portland the police have opted not to use tear gas vs right wing protestors. They've simply stood aside and watched as folks from the Proud Boys and similar groups drive around downtown shooting people with mace and pepper balls from paint ball markers. They've waited until after the proud boys et all finished their "demonstration", only then declaring an illegal assembly, and gassing the remaining protestors.

So you don't have to pretzel yourself into contrived hypotheticals. The police have made very clear clear that in your imagined case, they'd simply wave the rioters on with a smile, so long as they're against BLM.

I fear the answer you might well get will be "bullets". The idea that you need means to less-harmfully disperse political dissenters and opposition only holds if you want to live in a world where dissent and opposition are allowed. As an outsider, it seems very much like Americans (of all political stripes) are becoming increasingly intolerant of that.