Kyubey is incapable of refusing a wish*, so if someone wishes to not become a magical girl then they won't. They'd deem this a waste of a perfectly good wish and would attempt to dissuade people from using it, since it would basically do nothing -- including not progressing the fight against entropy -- but if someone managed to wish "I want to not become a magical girl," then they would not become a magical girl and nothing would happen.
That said, the act of granting the wish seems to be what creates the Soul Gem (note Kyubey's wording: "What is the wish that will make your Soul Gem shine," (as the gem is forming) "Your wish has prevailed against entropy"). Therefore, it is possible that this wish could be used to create a "blank" gem, with no one inside it.
* This isn't actually stated anywhere, but there's no other explanation, seeing as three distinct times across the franchise a wish has been granted that is, or seemed at the time to be, actively detrimental to Kyubey's goals (Madoka Kaname, Isabeau de Baviere, and Touka Satomi/Nemu Hiiragi/Ui Tamaki).