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But, also users don't lock their own thread now. I don't think they would care for automatically expiring their thread.
I think the best solution would be to set a default time of expiry for every thread created. OP would still have the option to re-open the thread if needed, otherwise it should remain closed.
If we were to talk about expiry times, I think 1 week would be good enough.
No it isn't. One week is enough only for threads starting in B&H board where most of the users normally asks dumb questions without searching anything at all wether that question asked and given answers several times. But for other boards where we start threads specially suggestions shouldn't enough one week. Those needs to be continued until it discussed all the aspects and all the scenarios that can be happened.
A much better fix is to tackle the issue at the source of the problem which is crap campaigns that are paying people to make the spam in the first place. Punish that and spam will stop or at the very least lower.
I really support most of your suggestions which will surely leads to make a better forum where we all can discuss and share knowledge about crypto field without having less spammers or shitposters. But not this one, we can't put expiry date for all the threads, to just stop the creation of redundend threads by spammers who cares only the number of posts to full fill there shitty campaigns. That will be a annoying factor for true members who really thinks about better conversation with other members (not spammers).
More viable option that I think as you already suggested is putting a barrier of 1Merit requirement for become Jr. Members. Why we are allowing to rank up newbies to Jr. Member just by wasting time with shitpost or oneline replies, but not contributing anything for the forum or other members. If we put that barrier, I'm pretty sure most of the spammers will not join bitcointalk or leave the forum since they couldn't ranked up easily and cannot join for bounties or signature campaigns to earn money. Less money, less spam.