df<- data.frame(a = c(1,2,3,4,5),
                b = c(6,7,8,9,10),
                c = c(11,12,13,14,15),
                z = c("b","c","a","a","b"))

a  b  c z
1  1 NA 11 b
2  2  7 NA c
3 NA  8 13 a
4 NA  9 14 a
5  5 NA 15 b

If column name is equal to Z, replace value with NA