I think there are several levels of severity in plagiarism and not all are equal and in some cases a permanent ban forever can be a little harsh (especially if it was just one silly mistake)
How do we determine it's a single case of a copy and paste? Let's say user X has 5000 posts and he is caught and reported for just 1.
I actually already posted a potential solution to this
Separately, there is an argument that bans for plagiarism should be delayed by a week, or 20 posts from the time a moderator discovers the infraction. The purpose of this would be too see if they will continue plagiarizing many times, or if they did something stupid on one or two posts. Someone who copies 5-10 of their next 20 posts is clearly not someone we want around, while someone who copies 1-2 ever might deserve some leniency, especially if they make generally insightful posts. This would help decide if someone will have *really* "learned their lesson" and wont make the mistake of copying content a second time.