How to avoid warning when introducing NAs by coercion

suppressWarnings() has already been mentioned. An alternative is to manually convert the problematic characters to NA first. For your particular problem, taRifx::destring does just that. This way if you get some other, unexpected warning out of your function, it won't be suppressed.

> library(taRifx)
> x <- as.numeric(c("1", "2", "X"))
Warning message:
NAs introduced by coercion 
> y <- destring(c("1", "2", "X"))
> y
[1]  1  2 NA
> x
[1]  1  2 NA

Use suppressWarnings():

suppressWarnings(as.numeric(c("1", "2", "X")))
[1]  1  2 NA

This suppresses warnings.

In general suppressing warnings is not the best solution as you may want to be warned when some unexpected input will be provided.
Solution below is wrapper for maintaining just NA during data type conversion. Doesn't require any package.

    as.num = function(x, na.strings = "NA") {
        stopifnot(is.character(x))
        na = x %in% na.strings
        x[na] = "0"
        x = as.numeric(x)
        x[na] = NA_real_
        x
    }
    as.num(c("1", "2", "X"), na.strings="X")
    #[1]  1  2 NA