At this time we are unable to finalize or reverse a segment of a split transaction.  As an alternative, we can finalize and then reverse the entire transaction (How do I refund or reverse a donation?) and add a new split transaction (How to enter in a split transaction) without that particular segment.  

Another alternative is to delete the segment of the split transaction.  We just have to be sure to adjust the other segments so that the total of each segment matches the total for the overall split transaction, or adjust the pledged, received, and non-deductible amounts on the overall split transaction.