The only way to say no is to say no.
You can lie about why the answer is no. You can explain why. But no is no.
The real question is this: is the value of the "lesson" you want to teach worth more to you than the dynamics of the relationship that can be forever damaged by your desire to teach this lesson, in this way?
It doesn't sound to me like you think the 600 is too much money. And it DID sound to me like it is a girl that you care about at least enough to be interested in her ability to manage her finances better, either for herself, or in context to her future relationship with you.
I would give, not loan, her the money, if it truly isn't a huge impact on your budget. THEN, and ONLY THEN, you will have the good will (or obligation from her) to discuss better money management. Maybe agree as a stipulation, not repayment, but that she enroll in a money management course, one you could take together in order to be able to 'discuss' the lessons.
Otherwise, demanding that your GF learn from the 'school of hard knocks' is not very loving, if you ask me. Standing by while someone you profess to love takes a fall in order to 'teach them a lesson' is truly 'tough love'. I'm not against the concept of 'tough love', when it comes to say, wayward children, etc. Sometimes, it IS necessary to learn lessons the hard way. However, BF/GF relationships are generally not permanent enough to withstand such stresses.
There are other ways to learn such lessons. Or more to the point, there are other ways for YOU to be the facilitator of such lessons, ways less damaging to a relationship.
This is presuming, as you suggested, that it's not really about the money. If you don't have that much money to just give away, then you shouldn't loan it out, either.
I will say this. I was in a similar situation once. I dated a girl that was always in need of financial assistance. She wasn't a poor money manager so much as she was trying to support a child on entry level jobs without assistance. That's what happens when you have a child out of H.S., stay at home, and find yourself by yourself raising a kid with no marketable experience at age 25. I never loaned her a penny, but I did 'give' her several hundred dollars over the course of the relationship; money I never asked to get back and never expected back. Ultimately, we DID break up, not because I was helping her financially all the time, but because I came to believe that that financial assistance with what she valued in me as a relationship the most. I had no problem with it being A reason to be together, from her perspective. However, it couldn't be THE reason, and I ultimately decided that it was increasingly THE reason. Not that I blame her, just, it wasn't a good basis for a long-term relationship.