Should be possible to send an update so future updates are only allowed if they're signed with a key cipher, then destroy the cipher. Basically put ransomware on it. Would need someone high up the chain to bypass whatever software checks are in place to keep someone from sending bad firmware up.

But the satellite orbits will eventually destabilize. If the satellite manages its own orbit, it could live out the rest of its planned lifespan, but de-orbiting then would be uncontrolled. Shouldn't cause too much risk, things happen already, but we try to set them down far away from civilization.

But if the satellite's orbit is managed from a central ground system, which I would assume is the case because that's the easiest way to reduce risk of collisions, then all the affected satellites would de-orbit at random in the upcoming months to years. Depending on how many are affected, could cause (probably) minor risk, but it would be in the news constantly for years, and the general public would probably lose favor of space operations, resulting in budget cuts and fewer satellites.

But IANARS, YMMV

Edit: oh cool, they're just abandoning them, so we get the mass unplanned deorbit route