You cannot have a completely free, unmoderated forum that won’t eventually get junked up to the point where it’s useless.
And pay to play will filter things a bit. I pay, which validates that I am a real user. I do the same here on FR, though no one notices. And do the same on other sites that I regularly participate in (where they also, like Twitter, identify real users). Not a big deal, the amount of money is small.