You can comment on posts. You can even comment anonymously.
However, I am under no obligation to treat your comments fairly or with an open mind. I will probably do so, because I do try to be fair and equitable in life, but this is my blog. Comment as you would if you were in my house, talking to me or to other guests. If you do so, we can have a good conversation, even if we disagree about stuff.
Comments that are abusive, obnoxious, or off topic may get deleted. I get to decide what is abusive, obnoxious, or off topic (again, my blog. You can get your own free blog from Blogger and have a grand time being abusive, obnoxious, and off topic in your own space). Occasionally, I will let abusive or obnoxious comments stand, just so the rest of the internet can see what a schmuck you are. Occasionally, I will let off-topic comments stand because they're more interesting than the topic.
I am less tolerant of stupidity in anonymous comments than in signed comments. More often than not, anonymous comments are the fast refuge of cowards and scoundrels.
Blogger does not currently seem to have a way for me to automatically close comments on old posts. Instead, it sends comments into a moderation queue. I only publish comments on old posts if I think they add interesting substance to what is already there. Comments on new posts can become part of a conversation, since people are still paying attention; comments on old posts are really not likely to be part of a conversation, but instead function more like footnotes.
For comments on old posts, I have to actively tell Blogger to publish them or delete them. Thus, I have to make a choice. More often than not, I will choose delete. If you really, really want your comment to appear on an old post, don't give me a reason to hit delete. Be interesting, thoughtful, insightful, etc. — be, in other words, worth publishing.
The most common type of old comments that come in are apparently from students who have been assigned to read one of my posts. I like students, and I'm happy to have them read what I've written, but all of the above still applies. Just because you have read a book or seen a movie does not mean I am then required to host your opinion of it or your opinion of what I have said about it. Add value to the post and I'll be happy to publish your comment and will be grateful to you for putting time and thought into it.