Sirius might use magic to punish them. He could hex them or use some other form of magic to make them feel the pain they inflicted on Harry.
The abusers could face legal consequences within the wizarding world. Sirius might report them to the proper authorities, like the Ministry of Magic. And if it's the Dursleys, they could lose their guardianship of Harry. Also, their reputation in the wizarding community could be severely damaged, as Sirius would likely spread the word about what they've done.
One possible consequence is that Sirius could make their lives a living hell. He has a fierce temper and a strong sense of loyalty to Harry. He might constantly intimidate the abusers, making them fear for their safety. If they are at Hogwarts, they could be expelled. And in the long run, they would have to live with the guilt and shame of what they've done, especially if the wizarding community shuns them for their actions.
In the fanfiction world, the abusers may find themselves ostracized by the wizarding community once Viktor reveals what they've done. If Viktor has friends or allies in high places, they could ensure that the abusers are held accountable for their actions. There could also be legal consequences within the wizarding laws if the abuse is of a certain nature. For example, if it involves dark magic or illegal forms of coercion, the abusers could be sent to Azkaban or face other forms of severe punishment.
If the champions decide to take matters into their own hands, they could use magic to teach the abusers a lesson. For example, they could jinx the abusers so that they are unable to harm Harry again. However, if they report to the school, the abusers might face a formal hearing. Depending on the severity of the abuse, they could be banned from certain areas of the school, lose house points, or even be permanently removed from Hogwarts. There could also be social consequences, like being shunned by other students.
Fleur might expose them. If they are students, they could face expulsion from Hogwarts.
The abusers might be ostracized by the community. Once Triton finds out and spreads the word, other sea - creatures or characters in the story might shun them. This could lead to them being isolated and having a hard time getting help or resources.
They could face expulsion from the wizarding community if the abuse is severe enough. The wizarding world has its own laws and rules, and abusing a young wizard like Harry would be a serious offense.
They might face magical punishment. For example, they could have their magic restricted or be sent to Azkaban if it's a really bad case. McGonagall would not tolerate such behavior and would push for the strictest punishment.
The abuser could face magical justice. In the wizarding world, there are probably spells or enchantments that can be used to punish wrongdoers. James Potter might use his connections to ensure the abuser is dealt with severely, like being sent to Azkaban or having their magic restricted.
Narcissa might be very angry with Harry's abusers. If it's the Dursleys, she could use magic to give them a taste of their own medicine, but in a more subtle way so as not to draw too much attention. For example, she might make their houseplants grow out of control or cause their food to spoil every time they're mean to Harry.
The abuser might face the wrath of the champions. They could be reported to the authorities in the wizarding world.
The abuser might face the wrath of the DA. They could be ostracized within the wizarding community. The DA members could use their influence to make sure the abuser is held accountable for their actions.