I faced this problem, I run a forums as subdomain and posted to lots of forms, many frauds content such as illegal contents, chines seller posted lots of fraud contents and after one year, I delete my forums.
i suggest you interact with your members like ask them questions or give them interesting to talk about. or maybe hire someone to be a moderator.
I added a forum to my wordpress site a little over a year ago and I have the same problems mentioned above. First getting anyone to post on it. Second to many spammers signing up for accounts. I have no idea how to solve the spam issue. Another thing I'm not sure of is what platform I should be using for the forum. The one I currently have is just a free wp plugin. but there is another on that I heard about smmachine I think. I'm thinking of giving that one a try. If anyone wants to check it out or leave a post here's the link http://tappowerhousemarketing.com/?page_id=60 Thanks for your input!