Spam also if the comments are closed

I’m receiving a consistent amount of spam daily. I used to have “Did you pass math” as spam filter, but it seems it didn’t work that well. I recently change spam filter and also tried to close the comments but same problem. I presume the robots saved somehow the way to send the spam for the old posts and regardless what I do, they still can do it (It’s a while I don’t post new posts). This is getting a bit insane though considering that my blog is definitely not that popular.

Does someone has any idea how I could fight the spam?

Thanks, chr