By most of the comments it is probably bad idea, with even 1% of chance to be accepted in any form - as forum rule, or just as a guideline for moderators. I was just thinking that if sometning is not reported in period of two years (and it was bad), it should stay in past - but I guess it is better to leave things as they are now.
I doubt people with thousands of posts will even try to do that, it is not only time consuming, but it is hard to remove such post if you do not know that it is copy / pasted from somewhere. In that one case where user is claim he bought account with plagiarism posts, admin is step in and check his log history which proved he is not real owner of account. I doubt we will see another case like that.