My wife wants to shave her head, but I'm not going to let her.
That seems a bit controlling. What's your problem with her shaving her head?
Because I love her naturally growing hair.
It's okay to tell your wife that you prefer certain things, but you should let her make her own decisions. Ultimately, her hairstyle is her own choice to make, not yours.
I'm going to make sure she can't shave her head.
Your wife is her own person and can do with her hair what she wants. I'm sure she would be fine with hearing your input, but at the end of the day, it's her decision. You should be encouraging and supporting of what your wife wants to do.