One big measure would be to have groups of users and something of a reputation system. New users would only be allowed to send and respond to invites for debates with others within their group(s). Once they have made, say, a certain number of arguments, they would be allowed to respond to invites from outside their group.
Thus, in order to successfully bring someone from outside the site into your debate, you'd have to invite them not just to debate but to join your circle, which feels quite different.