Technicalities are not an issue. It's a bit more murky when it gets to intent (was the user blatantly copy-pasting, or paying homage?), which is where mod discretion comes in.
It's quite murky. It's easy on random shit-posting baboons in pay-per-post campaigns. Say I had forgotten to include that source today. Would you have banned me? Note: Even though I wear a signature, I am not required to do anything thus making 1 or 1000 posts makes no difference. Financial motivation is a no-go here, and I most certainly did not intend to pass that as "my own work". So?
Hard to say without an example..