Can I say 没 to negate future events?
I've never heard of 没 being restricted to past actions. It is perfectly useful, as in 范冰冰明年没有结婚的打算.

I interpret these sentences as:

明年这个时候你还没有毕业呢 = next year at this time you will still not have graduated.

明年这个时候你还不毕业呢 = next year at this time you will still refuse to graduate.