There are many blogs including this blog closes comment section on older posts. I couldn’t manage to control spam comments on this blog especially on older posts. The reason why older posts are easy target for spammers is pagerank (PR). This is a keywordluv dofollow blog and most of the older posts gained some PR value. This is a shortcut to get a dofollow backlink from high PR pages. The moment one comment gets approved; you can see hundreds of new comments will flow to that particular post especially if the post gained high PR. So, the only way to control this is to disable comment section and stop worrying about it.
Some dofollow blogs disable comment section on posts older than 3 months, 6 months or one year. Personally, I disable the comment section on posts older than 30 days. This is not healthy for a blog in long run. We need comments as it helps our blog to grow in size of new contents added daily. The more contents left on comment section, the more it will help our posts to appear in almost any funny keyword searched on search engines. Content is so important and that is why blog admins are willing to give dofollow backlink to commentators as a token of appreciation.
Nowadays, I noticed that almost 70%-80% comments are made using keyword in name section. Ok, that is fine; it will help your SERP ranking. Not many will read full 300 words of blogpost but only the title and by reading other people’s comment before commenting just to get a dofollow backlink. This is unhealthy and is not productive to grow with unique contents. It will only cause furter damage to the blog. Imagine if a commenator’s link is pointing to some bad neighborhood site which can lead to penalty. Running a dofollow blog is certainly a risk that we all take if the commentator starts using names and links that are related to porn, gambling, etc.