A short pull-down list would work well: Spam, Harassment, Site ToS Violation, Thread/Group Rules Violation, etc.
This way you can automate rules like: if an article get's N spam or harassment reports it's put into the review queue and hidden until a Moderator can review it.
It would be nice if the report feature had a way to indicate if the problem is spam, content or whatever other issues people might have. You could have a threshold for spam reports to put the account in review and prevent of hide it's posts.