Getting Rid of rows in R based on if a specific column has a character in it

Using this sample dataframe in R:

1123 2 3 4
2789 1 3 5
C2124 1 44 6 
CS112 3 32 1

This data frame is huge, but this is all the info that's really needed to solve the problem I believe. How do I delete all rows in my dataframe where the first column (lets call it x and the dataframe df) starts with C or CS? I think my data frame is in a list format and not actually vectorized, because gsub doesn't work.