remove words based on first letter using stringr

Question

I want to remove all words that start with "a" in a string.

Input:

string <- "This is a sentence about nothing."

My attempt:

stringr::str_remove_all(string,"a*\\b")

output I got:

[1] "This is  sentence about nothing."

output I want:

[1] "This is sentence nothing."

I am not sure how to detect based on one letter but perform action(e.g., remove, replace) on the whole word. Any input is appreciated!