My forums are closed/private, so I don't have a SEO spam problem? - YES YOU DO!

It is not a good idea to believe that your private closed forum is free from SEO spam.  A spammer can still plant links and bate attractions so that your website users are attracted to roam off site for their benefit/advantage.

Closed forums are simply food for content trawling.  If Google has not seen your forum website content then such content is unique and good for publishing elsewhere.  You can loose your edge or niche into market simply because it gets published by a spammer elsewhere after they have copied or duplicated it.  It does not matter how pure you try to keep your website domain and it's content - it's simply published elsewhere with all the links and promotions attached.

Closed forums hold long term risks for your branding or what you stand for.  Closed private forums offer no protection from SEO spam.