You need to limit the number of posts anyone can make in a day or a week. I think one post a day, and maybe two replies would be plenty. Have a look at the Baytex (BTE) forum. One person posts 30+ times a day, every day, same nonsense every day, cut and pasted. It has been going on for years.

A simple limit of posts would be helpful, the spammers make the discussion worthless