It's up to forum owners to comply with the regulations. As a last resort they can always edit the database directly if what they must do is not possible via the UI. As discussed, Flarum has some built-in features and others are available through third-party extensions so there shouldn't be any excuse to an owner to not do the right thing.
If the owner is not responsive, you can always try reaching out to their host as well. If the forum is hosted by FreeFlarum this should be simple enough, they have email and their own support forum. If it's a self-hosted forum it might be more challenging to figure out where it's hosted.