The Marauders didn't stop bullying Snape after the prank. It actually got worse.
A lot of people are surprised to realize that the scene in Snape's Worst Memory happens after the werewolf prank. When first reading OOTP, people generally assumed that SWM showed escalating tension between the Marauders and Snape that up led to the prank. But in DH, we see Snape and Lily talking about the prank before SWM. This means that the Marauders are still singling Snape out and targeting him after prank. Why?
My theory is that the bullying actually got worse after the prank. Because the only way to hold their friend group together was for the Marauders to double down and rally around blaming Snape for what happened.
Think about it: How did that incident not tear them apart? Sirius not only exposed Lupin's secret – he also attempted to use Lupin as a weapon against Snape, and he could have gotten James killed in the process. That's a huge betrayal.
But Sirius isn't mature enough to take responsibility for it. Lupin isn't self-confident enough to confront Sirius about it. "James would-consider-it-the-height-of-dishonor-to-mistrust-his-friends Potter" isn't going to be the one to lay blame on Sirius or break up the group. But it's too big an issue to ignore. The only way they can get over this is to put it all on Snape. It was just a joke, and Snape is an idiot, and James is a hero.
If you compare the two incidents that the books show us of the Marauders bullying Snape, you can see that totally different dynamics are driving the bullying. This shows how and why the bullying got worse after the prank.
The first bullying incident we see is on the Hogwarts Express, when James and Sirius engage in verbal bullying of Snape, with one small attempt at tripping him up as he leaves. This bullying is a form of bonding for James and Sirius and forms the basis of their friend group. This is an example of bullying driven by Peer Group factors (source), and this sort of bullying is generally done to:
to attain or maintain social power or to elevate their status in their peer group.
to show their allegiance to and fit in with their peer group.
to exclude others from their peer group, to show who is and is not part of the group.
What we're seeing here is that the soon-to-be Marauders are in new environment and they're defining their peer group and establishing social hierarchy, trying to establish their status. The Marauders continue in this pattern of Peer Group bullying throughout their school career, as evidenced by the detention records Snape has Harry transcribe in HBP. The Marauders seem to have thrown out hexes in a scattershot way to establish superiority over other students and look cool. This casual, incidental sort of bullying is likely what Snape experienced for the first several years of school.
But what we see in SWM isn't bullying to maintain Peer Group dynamics. This bullying isn't just flinging a single insult or a clever hex. James and Sirius hunt Snape, they deprive him of his wand and ability to escape the situation, and they repeatedly hex him until Lily (temporarily) stops them. This incident is extremely personal. This is an example of bullying driven by Emotional factors, and this type of bullying is done when the bullies:
have feelings of insecurity and low self-esteem, so they bully to make themselves feel more powerful.
don’t know how to control their emotions, so they take out their feelings on other people.
may not have skills for handling social situations in healthy, positive ways.
What we're seeing here is all the fraying edges of the Marauders' friendship. Sirius has just damaged their group, but he can't apologize or address it without accepting blame, so he has to take his emotions out on Snape. Punishing Snape is a way to exorcise his guilt. And it's actually imperative that he bully Snape into silence, because he is the one who has revealed Lupin's secret to Snape and put them all in jeopardy. Lupin can't confront Sirius about the betrayal of trust, and likewise he can't confront his friends here. Not only does Lupin not have the emotional security for handling this situation, he also can't risk putting himself in front of Snape in this moment, lest Snape scream "Werewolf" instead of "Mudblood." James is here trying to work through his own insecurities – in bullying Snape he is defending his friends, but James is also trying to get Lily's attention. James offers to change his ways if she'll give him a chance, because James needs to reassure himself that he is chivalrous, that he is a hero.
Looking at the way the bullying dynamics change and escalate in those two scenes, I think it’s clear that Lupin’s line, “Snape was a special case. I mean, he never lost an opportunity to curse James so you couldn't really expect James to take that lying down,” is an understatement.
Snape was a special case because he knew Lupin’s secret, which would always make him a potential threat. The Marauders would always take any opportunity they could to reinforce that Snape was powerless to do anything to them. And they’d continue to take out all their emotions about the prank on Snape rather than confronting each other.
