Well what's stopping her is the part in the shikai description where she has to touch the end-destination of her portal with her sword beforehand. To put a rock in someone's brain, she would have had to already cut their brain with her sword. Kinda makes putting a rock their redundant.

I will admit though that there's room for clarification in the power, and making it so that a living being can't be the end destination is a good additional safeguard to the power.