How do you want to enforce this? Give Mods more work?
Here is a radical suggestion. Ban all Bitcoin loans.
We're all (supposed to be) adults. And even if we're not adults, it's a good life lesson to learn with small mistakes before you make big mistakes.
If someone wants to give out a loan, that's entirely their own decision and risk. I've never loaned or borrowed Bitcoins and I don't plan to do so, but if someone else wants to, let them!
Plus, the loan-topics and Trust with it are great to read while eating popcorn!