I involuntarily made a "rule" like this, and basically... my rule was "register your OOC account first, it's mandatory." It was the first rule in the rules, and so many people would ignore it and jus tmake an OOC first. So, I could tell they didn't read the rules...
However, it later became clear that was not always the case. A lot of people did read the rules (I could tell after they immediately mentioned something else that was in the rules), but there was an issue with either skimming or "blind formatting," basically the eyes naturally scroll over something. So, I changed the formatting a little, and I've had less issues since. Personally, I'm so accustomed to signing-up all uppercase because that's what I always do (I don't require it at my site, I just like how uniform it is on the drop menu XD) that I accidentally did that at a site that required lowercase even though I read the rules! It was an immediate "DOH!"
So, this "trick" is not fool proof no matter what.