I've actually had a quite similar tought as the OP for a long time. In my country there's a time limit for each crime.
This is the wiki page (sorry, it's in dutch):
https://nl.wikipedia.org/wiki/VerjaringFor example, a traffic violation can only be punished if it's brought before a court within 1 year after the crime has been committed. Other crimes carry an expiration period of 5, 10, 20,... years, but only crimes like commiting genocide don't carry an experiation period.
If this expiration time has passed, it can still be proven that a crime has been comitted, the person is still guilty, but the court finds that it's no longer a good idear to actually punish somebody for a crime they committed so long ago (given the fact that they didn't re-commit the crime afterwards).
In forum terms:
it's Theymos's forum, he's the admin and by having an account on bitcointalk you'll have to follow his rules. That being said, i think we can still make an argument towards Theymos and maybe make him changes his mind.
I, for one, would like to see *some* crimes "expire". Not because i have an alt account that's being banned, but because i think banning people for things they did years ago will make good community members leave this forum, making it a worse place. If somebody commited 1 act of plagiarism 5 years ago, but never did anything wrong after this 1 act, while still making hundreds of helpfull posts, would it still be a good idear to punish him/her (even with a sigban)? Offcourse, if he plagiarised 5 years ago, and re-commited plagiarism 1 year ago, he can't be punished for the crime he comitted 5 years ago but he CAN be commited for the one 1 year ago.
I think, legally, it might also be a good idear to remove old, plagiarised posts by a red warning so no offending content can be found on bct, and an offender should still receive a public or private notice that what he did was, in fact, a crime, but he's not being punished because the crime expired....
Just my inner toughts here... Open for debate..