I agree with ninjaa. If ranks are taken out, the competitive people will either stop playing here or just play the same way as before. While ranks matter to some, it's more their gamestyle than anything else. And honestly, you can't get angry at these people. They are playing prophunt the way it was designed, without breaking the rules. And I'm fairly sure they're having fun or they wouldn't be playing.
One idea I have had for a while, but would be more difficult than the other two options or either having ranks or not having them, is to make a different rank system. One similar to dodgeball. It wouldn't keep track of your hours played, total kills, kill/death ratio, just your best. For example, the top kills in one map on say Oasis. It would show the best of each players' points by the end of the map and rank them. The upside, in my view, to this is that people can mess around and have fun without worrying about what it will do to their rank, but there will still have a competitive side some people like. Just an idea.