The "sensible" solution for plagiarism would be this.

a button on every post labeled with" Click me NOW" , takes you to another page where you need to quote the original text, the two texts along with the timestamp run in simple query that checks the integrity of the report by matching the two texts, if it matches , BINGO ! that account gets INSTANT BAN for a limited period of time which equals to his lifetime.

and yes , the button should be available to everybody including newbies so that they can ban higher rank members who copy and paste.

I can tell you this is going to be a 100% plagiarism free forum, as everybody will be watching over everybody else.

and this to me is the ONLY "Sensible" punishment for somebody who steals the words of others and present them as their own.