Comment by Cthulhu_
7 hours ago
I've tried to look up whether there's a reserved username list option for these tools but either there isn't one or I suck. One option is to set / reset the NAME_REGEX configuration (or env var?), individual distributions could do this.
Silly idea: just create those users manually without giving them a home directory? The names won't be available anymore. You could write a short script wrapping useradd (and userdel? I honestly haven't deleted a user in years and can't remember if that's the correct command) and define your own config file to contain the current set of names managed by the tool.