After Harry Potter stops the bullying, the bullies could have a change of heart. Harry's intervention might make them realize the wrongness of their actions. However, if they don't, they could be ostracized. Their actions would be public knowledge, and other students would be less likely to befriend them. The school might also impose stricter rules on them to ensure they don't repeat their bullying behavior. They could be made to attend anti - bullying classes or do community service within the Hogwarts community.
In the fanfiction, once Harry stops the bullying, the consequences for the bullies could be far - reaching. Firstly, they may lose their status among their own group of friends if those friends start to see the error of their ways. Magically, there could be a sort of 'karma' spell that backfires on them, causing minor inconveniences like their wands malfunctioning more often. Socially, they'll find it hard to gain the trust of others again. And if they continue to be unrepentant, they may find themselves in a very lonely and difficult situation at Hogwarts, with most students avoiding them and the teachers keeping a very close eye on them.
The bullies might face some form of punishment from the Hogwarts faculty. Since Harry is well - respected, his report of the bullying would likely be taken seriously. They could get detention, lose house points, or even be suspended. Also, they may face social isolation as the other students might look down on them for their mean behavior.