I'd say that ceasing the policy of letting people replace a replay would be a good enough measure to make it harder to join.
If you don't like a particular replay in the app, the person isn't ready yet since those were most likely their best 3 replays up to date. The app has to be good as a whole.
And if your not sure about someone that means his replays weren't convincing enough, so he'll have to improve, even if just for a bit.