The fundamental reason that people say ACAB is because the people that are trying to do good things still enable the shitty ones. That is, they fail to act when they see shitty cops; the ‘good’ cops don’t police the bad cops, and that makes them, in turn, bad cops themselves, because it allows bad behavior to be normalized. The relatively few cops that won’t go along to get along quickly find themselves left high and dry; the get the worst duties, don’t get backup in a timely manner when they need it, don’t get promoted or end up being demoted, rack up a long list of bullshit infraction of departmental rules, and so on, until they get forced out.
People talk about reforming the system from within, but it’s a top-down problem, and the police unions are either directly involved or, at the absolute minimum, are complicit by working to protect cops that the union knows are corrupt.
The serious answer is that you need to have outside control over the process. Cops need to be accountable to someone other than themselves. Civilian review boards are a good first step, but you need to make sure that the civilian review boards have real teeth, and that they don’t get captured by the police (e.g., the people on the review board all being family members of cops). Note that police unions and officers have long opposed civilian oversight boards, because that removes part of their power.
You also need to ensure that prosecution is always handled by an independent agency. A local DA will need to continue to work with police, so it’s against their interest–and hopefully also in the public interest–to create a hostile environment where the police think that the DA is ‘against’ them.
You also need a way to limit the power of the police union. A police union does serve a real purpose, in that it should insulate cops from acting as directly political agents. OTOH, it also protects the worst cops out there, and makes it nearly impossible to quickly get rid of someone that’s clearly unfit for public service of any kind. I don’t know how to do the latter without also undercutting protections against the former.