You are correct, anyone can post to any thread they want. However, this board also has a function that allows you to ignore a user. I know you are familiar with this because you learned about it on the same thread I did.
So, my recommendation is to ignore a user if you choose not to read their posts. It's a very effective tool. (for those who do not know, go to the users profile and the option is there to ignore, you can return to their profile if you'd like to remove the ignore request)
