Should topics of permanent banned plagiarisers be locked after bans?

[SquirrelJulietGarden]

My proposal

Topics of members who were permanent banned should be locked after their bans go effects.

If topic creators won't be able to come back to post, reply, update OPs, is it needed to keep their topics open for others? A few explanations for my proposal.
- Plagiarisers: are mostly spammers and their topics mainly have low quality.
- Posters participate in low quality topics are mainly spammers.

The warning can not be enough to stop spammers.

Warning: this topic has not been posted in for at least 120 days.
Unless you're sure you want to reply, please consider starting a new topic.

Topics of permanen banned members can be kept to open for 120 days and will be locked if no one make posts in for 120 days.

[Pmalek]

I see no reasons why topics of members who were found to copy/paste should be locked. There have been many cases where users plagiarized several years ago and were banned for 1-2 plagiarized posts, but contributed a great deal to the forum after that. A member can copy/paste 1 post and have 100 great and genuine ones. It goes the opposite way too.   

[Upgrade00]

A ban is a punishment for rule breaking, not an indication of a user's contribution or spam index

The effect is barely noticeable (i doubt majority low-quality thread are created by plagiarizers).

I agree with this, unbanned users rarely go back to lock or update their threads, so locking those of banned users would not have much of an effect in reducing spam mega threads, neither would it be targeting the actual issue.
If one feels a thread has become superfluous: they can report it to the mods (whether or not the OP is still active) and include all relevant information in the report comment.