I have another Raspberry Pi sitting next to my desk, with a keyboard and a tiny screen, dedicated to systems administration. My user on this machine has an SSH key that on every machine logs into an account with sudo access. To revoke a user key, I run a script from this machine that logs into each host and updates sshd’s RevokedKeys.

I have no mechanism at the moment for revoking host keys, which is a harder problem to solve as it would involve updating a number of laptops, phones, etc. that may not be powered on at a given time, but that’s less of a problem since if I knew a host key had been compromised I wouldn’t be logging into it anyway.