I noticed a bunch of mega large topics that are mostly used for spamming and have little value to provide in forum.
Is there any policy and rules that that enables moderators to lock this topics after certain amount of time?
If the topic is "out of date" then yes, it can be locked. For example: Will Bitcoin reach X by X date. Once that date passes, the topic should be locked. Some "mega topics", especially those with polls, are arguably still useful/worthwhile, even if the discussion usually deteriorates over time. If you come across outdated topics then you can always use the report to mod function to get these topics closed, as many users already do. But please don't report topics just because they are old or already have hundreds of replies, this would be an abuse of the report to mod function.
Sadly, there is a huge incentive to post there: signature campaigns with high quota.
I have some difficult to reach max posts per week, and I believe most users have this problem.
So people just go to a megathread topic and spam there... it is easier than to create a new topic with some new info
The issue is two-fold really. Firstly as you put it due to signature campaign quota, secondly it's based on lack of new topics (or good topics).
If users would like less megatopics and more diversity of topics, then I recommend creating new topics and becoming part of the solution to the problem
