Maybe if you rename the subforum to something very specific then people will know better. Instead of 'this group', say 'dnncommunity.org'. Or rename it to 'Issues about dnncommunity.org site'. or 'About this site (dnncommunity.org)'.
Otherwise you will just be moving posts all the time. These users are probably unaware that their posts have been moved. I notice to them that they posted in the wrong forum can help.