The trial is over and Tom Robinson has been convicted. But Bob Ewell is still furious with Atticus for opposing him and for making him look like an ignorant coward in court. So, Bob spots Atticus in the post office and spits in his face, knowing that Atticus will not fight back. Atticus says he is "too old" to fight back but he also restrains himself because he would rather have Bob take his anger out on him than on the Ewell children. At the beginning of Chapter 23, Miss Stephanie notes how Atticus simply took the abuse from Bob:
Miss Stephanie said Atticus didn’t bat an eye, just took out his handkerchief and wiped his face and stood there and let Mr. Ewell call him names wild horses could not bring her to repeat.
Bob Ewell shows his cowardice because he attacks someone who will not fight back. It is also alleged that Bob is abusive to his children, many or all of whom can not fight back.
When Helen goes to work for Mr. Link Deas, she must walk past the Ewell's home. But she starts going out of her way because Bob throws things at her as she passes. Link Deas warns Bob to leave Helen alone. Again, Bob is attacking someone who will not fight back. Bob then starts stalking Helen.
She turned and walked on, and Mr. Ewell kept the same distance behind her until she reached Mr. Link Deas’s house. All the way to the house, Helen said, she heard a soft voice behind her, crooning foul words. (Chapter 27)
Link Deas warns him again, more sternly this time, and Bob stops harassing Helen. Bob backs off when he feels legitimately challenged.
Bob shows his cowardice most overtly by attacking those who can not, or will not, fight back. In the end, Bob attacks Jem and Scout. At the end of Chapter 29, Heck Tate sarcastically talks about Bob's guts. Tate is clearly emphasizing Bob's cowardice, saying Bob would not attack Atticus or his children in daylight:
"He had guts enough to pester a poor colored woman, he had guts enough to pester Judge Taylor when he thought the house was empty, so do you think he’da met you to your face in daylight?"