One option is to simply avoid reading or engaging with Rias fanfiction. If the OC hates it, there's no need to subject it to that content.
If the oc hates it but still wants to give it a chance, they could try to find a different take on the rias fanfic. Maybe look for ones that are more in line with their own values or interests. For example, if the oc is interested in action, find a rias fanfic that has more action elements rather than just the typical romance. Another thing to do is to try and write their own version of a rias - related story that they would like, incorporating the elements they enjoy and leaving out the ones they don't.
The OC can simply avoid reading any more of that fanfic. There are plenty of other stories out there in different fandoms or even within the 'High School DxD' fandom that might be more to their taste.
It could be that the OC creator simply doesn't like the typical tropes or storylines often found in Rias fanfiction. For example, if the fanfictions tend to be very formulaic or over - use certain clichés, the OC might be a symbol of a break from that. Also, perhaps the OC has a specific backstory or purpose that is at odds with the general narrative of Rias fanfiction.
Maybe the OC has a completely different personality or set of values from what is often portrayed in Rias fanfic. It could be that the OC has had bad experiences related to the typical tropes or themes in those fanfics.
Maybe it's because of the different power systems. Naruto has chakra and his ninja abilities, and Rias has her devil powers. If in the fanfiction these power systems are not properly integrated or if one overshadows the other in an unappealing way, Naruto could be depicted as hating Rias. Say, if Rias' devil powers are made to seem too overpowering and it goes against Naruto's sense of fairness in combat, it could lead to such a scenario.
The writers could stop writing that type of fanfiction if they respect Ash's 'feelings' (assuming he were a real person with real opinions).
She could also engage with the fanfiction community and share her views. Maybe by expressing what she doesn't like about the current Hermione fanfiction, some writers might take her ideas into account and create something more to her taste.
If Paul hates Bella fanfiction, he can simply avoid reading it. There are plenty of other types of literature or fan - made works out there that he might enjoy more.
One option could be to simply avoid reading it. If it bothers him that much, he doesn't have to engage with it at all.
He could also try writing his own fanfiction. By creating his own version, he might gain a new perspective on Hermione fanfiction and perhaps even start to like it. Or he could engage in discussions with fanfiction writers to understand their intentions better.