Why do you want to stop volunteering? Was the volunteering that you did just to get you into medical school, or did you actually enjoy giving your time to others for free?
I was actually asked this very question by my mother this morning, and I told her that I would continue until I left for medical school. She was disappointed as she wanted me to spend as much time with my husband as I could before medical school started. I actually enjoy volunteering and giving of myself to others, and I wish I did more before instead of wasting half my life. So, I plan on continuing right up until I head off to medical school.
Of course, in medical school we won't have time to do anything but study, but I will do volunteer work after medical school where I can actually make a difference (as someone who can practice medicine) rather than just volunteering without skills as I do now.
And, it's probably true. The school won't care if you quit volunteering as the whole admissions process can be gamed by those who play the system right.