One could limit share of hash power. Perhaps by IP address. Or through blockchain links to previously-solved blocks.

The Tor Network, for example, has mechanisms for excluding bad participants. There's the bad exit flag, for relays that snoop traffic. Relays that harvest onion hostnames get banned. So do relays that attempt traffic analysis.

There's even a policy of discouraging new relays in commonly-used AS. There's no exclusion mechanism. But I don't see why there couldn't be, if there were too much concentration.

Edit: See https://trac.torproject.org/projects/tor/wiki/doc/ReportingB...