anyone have a way that an ordinary Slack user can mute/block/no-longer-see another user? not talking about what a channel mod can do, or team admin. I've been really surprised Slack didn't already have a built-in way to do this.

I don't think it's possible. Slack is built for teams in the same workplace - there's usually no need to block or mute your colleagues and if there is, then HR is involved.

Is there a problematic user that the admin team should try to do something about? If so, you can DM one of the admins and we can discuss what to do...

The Code of Conduct — and list of Admins — is here https://goo.gl/VsJ8q8

thanks guys! but think you both missed the intent of my question. not the use case

As Anna said, Slack is designed for work teams so, no, individual users cannot mute other users. If you are finding certain users annoying enough that you want to block them, talk to the Admins about. If your use case is something else, feel free to explain here @groglogic