C# text file search for specific word and delete whole line of text that contains that word

Basically I have a text file that I read in and display in a rich text box, which is fine, but I then want to be able to search through the text for a specific word and delete the whole line of text that contains this word. I can search through the text to see if the word exists or not but I cannot figure out how to delete the whole line. Any help would be great.