Hmm, apologies for the mixup. After reading through the CC licenses again it seems that people would still be able to share under that license, but attribution will always go back to you. Not what you’re looking for.
When you write code, you own the copyright to it. If you put your code up without a license, then all we have is your copyrighted code. As such, the code will be read only and belong to you. It wouldn’t stop someone from downloading it and doing their own thing, but they shouldn’t if that’s the case.
That’s not open source, but would satisfy your needs. If you can, check with the organisers to be sure it’s OK.