I would, however, favour a one-warning system, not 10 points - those that plagiarized after they got the single warning, should only get less than a ban in very special occasions (e.g. if they really developed into a very valuable member, e.g. people posting good Bitcoin tutorials and participate in development). In this particular case I think a community vote could be part of the "recovery" process. But I would require at least 500 self-earned Merit points (so I would still not qualify, for example) for that, because it's a pretty labour-intensive task for moderators to evaluate these cases.
Sounds fair to introduce a one-time warning and if the user is caught doing that again he will receive his ban. And honestly, I wouldn't have a problem if someone gets banned like that because he was warned and totally aware that a repeated offense would result in a ban. The positive effect will be that plagiarizing for sigspam would be useless because they seem to do that regularly and normal users doing that by mistake will get a reminder to quote or like other sources.
Requirement to receive a warning should be at least Jr. Member rank that mods can still nuke spambots.
For the voting I can imagine that the user can add a poll to their ban appeal and the result of that poll will have an unofficial character, especially if the global moderators don't know much about the user because he was active in local boards.
Such polls can help to figure out if the user is useful for the community or not.