This is not currently an option. Currently, you would have to adjust their total up to zero if a member goes negative.
By what mechanism are you looking at for zero prevention?
The only two I can conceive of:
1) A user receives an item for which they don't have as many points as the item costs, in which case they go below zero and the system automatically adjusts the user back to zero.
2) A user doesn't have as many points available as you'd like to charge for an item and so the system prevents the addition of said item.
Neither of those solutions seem great to me, maybe the first seems okay,
I guess, but a person spending those points is still spending them, and should, IMO, have to earn that same number of points back.
A user going zero isn't really a problem when you consider that zero is just an arbitrary number on the number line.
The only sure-fire way to prevent going zero (without using adjustments) that I'm aware of in DKP is a pure percentage based item cost system, like
this one.