As already noted, this is very common...unfortunately. People either don't read the rules or don't care, don't think it applies to them. And then scream if you disqualify them accordingly.
I've been in Beth's position a few times, and have seen the same thing more times than I care to think about. Not just in contests, but just the everyday rules and guidelines of a thread type forum or chatroom. You make it very clear what the rules are and plenty people simply ignore it. I've banned quite a few...
For example right now, I set up a thread for people to show off images featured in whatever various groups they have joined. So far it looks like everyone has been posting just their featured work from various groups, but I also stated in the few and simple rules at the top "3 per post". Someone immediately posted 6...I haven't researched it thoroughly, but I don't think I can delete the 3 extras, or the entire post but would love to be able to. Sorry, you ignored the rules...
I don't feel bad about that. I'm not a huge fan of lots of rules, if I set up a thread or contest I'll try to keep it simple, but I expect participants to follow those guidelines and when I see infractions, I don't mind deleting them or whatever else needs to be done to rectify it.