nzcoops is spot on with his suggestion. I posed this question in the R Chat a while back and Paul Teetor suggested defining a new function:
`%notin%` <- function(x,y) !(x %in% y)
Which can then be used as follows:
foo <- letters[1:6]
> foo[foo %notin% c("a", "c", "e")]
[1] "b" "d" "f"
Needless to say, this little gem is now in my R profile and gets used quite often.