remove all instances in first table that exist in second table

negative join

library(data.table)
library(dplyr)

a <- data.table(x = c("A", "B", "C", "D"),
                y = c(1, 2, 3, 4),
                z = c("good", "bad", "bad", "good")) %>%
  setkey(x, y)
b <- data.table(x = c("B", "C"),
                y = c(2, 3)) %>%
  setkey(x, y)

c <- a[!b]