We see that towards the end of Episode 146 and in Episode 147, Luffy and Zoro don't fight back against Bellamy although they could have easily beaten him. Why is that?
As a guy on Reddit put it:
It goes back to Shanks when he was in East Blue. The bandits did the same thing to him and he didn't fight back either. Zoro was simply following the command of his captain. Luffy said don't fight back so he didn't fight back. Luffy however wasn't fighting because there was no need to fight them. He later however did "fight" Bellamy when his friends were attacked by Bellamy, similar to how Shanks and crew fought for Luffy when the bandits attacked Luffy.
Hence, Luffy didn't retaliate because:
Luffy was inspired by Shanks' decision to not fight back in the past without a reason
Bellamy wasn't standing in the way of Luffy's goals or had hurt any of his friends (until later in the series). Luffy being the kind of guy he is, only picks fights when his friends are hurt or somebody stands in the way of his goals.
He seemed to lose interest in Bellamy when Bellamy claimed that "The pirates' dream era is over" and started depreciating pirates who believed in the City of Gold, The Emerald City and the great treasure of One Piece.
As for Zoro, as stated, he was simply following the captain's order.