The best way to shut down a "police are racist" person is this.... Say that police must be sexist because the vast majority of arrests are men. They'll say "that's ridiculous, it's because men commit more crimes" Just stare at them until they figure it out and watch their head explode. They might say "well it's because minorities are oppressed". You can just reply with "so men are oppressed?" Etc.... anything they say at that point backfires. If they bring up historic racism, you can point out that the 50/13 statistic has been unchanging since the 1950s. And the country is far less racist now than it was then. The blame is not on people of a given race because they're inherently criminal though. But the cause is certainly not "muh rayycism".