Shadowbanning. Either outright ban someone and let the community know why, or don't. That's it! Transparency and honesty are the way to go. Arguably this is more of a moderator/admin morality issue than anything else, but still.
This I disagree, I believe it's useful to let trolls and spam bots talking to themselves, alerting of a ban will only make them to take measures to circumvent the block. The problem would be mod abuse, not the tool.