How do I exclude all values in a row where the first instance of this value occurs? (R Dplyr)

I have a dataset, df

Subject

Hi
hello
RE: Hello
RE: How is work
No
Ok
RE: What time are 
Hello RE: are you

I would like to exclude all rows where the first word is RE:

Subject

Hi
hello
No
Ok
Hello RE: are you

Here is the dput:

 structure(list(Subject = structure(c(2L, 1L, 5L, 6L, 3L, 4L, 
 7L), .Label = c("hello", "HI", "No", "ok", "RE: Hello", "RE:   How     is work", 
 "RE: What time are"), class = "factor")), class = "data.frame",       row.names = c(NA, 
 -7L))

I have tried this:

   df %>% 
   filter(Subject!= %RE:)

I do not know how to formulate the code so that it will exclude only if is the first instance within the row.