I tell the kids in my adopt an IM talks this..

There will ALWAYS be someone faster than you, and always be someone slower. Don't sweat the details. Run your race, and do YOUR best, for whatever that is on that day.
But don't let the thought that you aren't fast enough keep you from trying something that you just may love!

Personally I just shrug off those that show up with no real training under their belts. I figure they will learn. Look at Lance last year in the NYC marathon. Yeah he finished in under 3, but just barely, and not without a lot of pain. I think it was his ego alone that got him through that race. He had pacers, and fans along the entire course. But even with his athletic (ya think?) background I knew what was going to happen after he said his longest run was like 2 hours. I want to say 15 miles or something? I was like "ut oh... this is gonna be a train wreck"... And I was there, so I can tell you- he was in PAIN at the end. Muscle memory can only get you so far.
But they don't really bug me. As long as they take the time to read the rules.
If your not gonna take the time to train, at least take the time to read the rules!!