If the three conditions you outlined were met, I would recommend:
- posting a link within the thread to the original source
- dropping the node rep to 0 if it were positive (or leaving it as-is if it were negative)
- assessing a 10 XP penalty to the OP
- appending "(plagiarised)" to the title of the node (not the whole thread) (yes, I know it is currently possible for the OP to edit it)
I think it is important to target only those nodes that were plagiarised - not every post ever written by the OP. This would specifically discourage the negative behavior and make the Monastery's response feel less like an unrestrained personal attack. It may also make it less likely that the OP simply create a new account and continue the behavior under a new nick. If a significant pattern of plagiarism was found (as in this case), then a graded progression of consequences (including locking out/deactivating the OP's account) could be considered.
Finally, I agree with what Joost said: reposting interesting questions or answers from other sources is (in general) fine "as long as the content is interesting (and prefererably attributed)" (emphasis mine).
Just my $0.02