I'd like to see linking to paywalled sites discouraged, but banning them might be too inflexible. Perhaps a change to the submission form that suggests that the submitter looks for alternative sources first, and appends "(paywalled)" to the title if they opt to proceed.

Paywalled mass-media articles (e.g. WSJ) are very rarely worth submitting. Articles in paywalled journals (e.g. Nature) where the abstract is accessible may well be worth it. But it comes down to judgement.

Finally, I agree that (for the type of topics that HN concerns itself with) free access to information should be encouraged.