I don't really think DR warnings are going to do much of anything. If BB wants to show that they are really against what the houseguests are saying, they need to show that they aren't messing around.
I don't necessarily think this has to mean throwing a houseguest out of the game, at least not straight away. Put repeat offenders on slop for a week, and then if they continue, maybe restrict their game options (ie ban them from competitions), and then in extreme cases, boot them out the door. At the moment, there's absolutely nothing stopping houseguests spewing as much venom as they want, because they know that nothing bad is going to come of it. Hell, if really lucky, you might get a second season, a stint on the Amazing Race with your showmance and your very own web series...