Changing table values based on last occurrence

Hi,

I have a pivottable with three columns. The last Y values of a and d are zero, therefore, I would like to change all the Z values corresponding to a and d to be 0. Is there a way to accomplish this in excel? I can get the index of the rows where 0 appears but am not sure what to do after that.

X Y Z
a 4 0
b 3 3
c 5 5
d 6 0
a 2 0
e 1 0
f 3 3
c 5 5
a 0 0
s 2 2
b 2 0
d 0 0

Thanks,
John