It is worth thinking about the reasons why Sammy chose to quit. Of course, the obvious target of his rebellion is Lengel, but what really causes his rebellion is the way that Lengel treated the girls and chose to embarrass them. This leads us to think that, although Lengel, as he makes clear, has every right to challenge the girls, Sammy, by quitting, is actually questioning the standards that allow a man like Lengel to embarrass girls. Sammy seems to be suggesting that there is some form of a higher standard of common decency that dictates you should not deliberately embarrass others, especially in front of a crowd. Thus we could argue that Sammy is actually rebelling against his society's rules of conduct. His act of quitting is symbolic of a challenge against the values of his society as he stands up for what he thinks is a higher set of values.
Even though he recognises that he will "feel this for the rest of hsi life," and that his parents are dependent on him, as Lengel reminds him, Sammy still feels this is an action he has to go through with:
... but remembering how he made that pretty girl blush makes me so scrunchy inside I punch the No Sale tab and the machine whirs "pee-pul" and the drawer splats out.
Sammy acts on instinct to some higher code of ethics that condemns the lesser form of conduct that he sees in those around him, and thus quits, daring society's censure for his actions and his deeper act of rebellion.