What if someone wishes to not become a magical girl in Madoka Magica? In Puella Magi Madoka Magica, if someone wishes something to Kyubey, they will become a magical girl (and in the end, a witch), but the wish can be anything like money, power, godlike powers. However, what would happen if they wish to not become a magical girl instead? (Perhaps only Madoka knows)

I was thinking if Kyubey would explode or refuse the wish, but what if he didn't refuse?
Kyubey gets these girls to sign a contract. This contract includes the provision that they become magical girls and fight witches (with the hidden eventuality that they will become witches themselves). Granting the wish is Kyubey's part of the contract. If someone does not agree to the contract in full, including the part about becoming a magical girl, why would Kyubey grant them a wish? In other words, not agreeing to become a magical girl in any way constitutes not agreeing to the terms, refusing the contract.

You must also remember that Kyubey intentionally offers the contract at specific moments when his target is most likely not to refuse. This is exemplified in Mami's case, explained in the Puella Magi Wiki:


  Mami made her wish after she was in a car accident with her parents. She was most likely very badly wounded, as her parents died on the scene, and Mami only survived due to making a wish with Kyubey, who appeared at just that moment, leaving Mami little time to consider her wish.


It's unlikely that Kuybey would approach someone at a time when they would either refuse to sign the contract, or who would in any other way not use the opportunity to wish for something important to them.