Presumably your phone has some tool (from your service provider, or linked to your phone app) that checks numbers against some external register. Whatever had that "report" option will be the thing using this tool, presumably. So other than what you've done, all I can think of is using a different app that doesn't have this integration (every app on your phone can be replaced, including the phone dialer). But that will just stop it having a spam flag on your phone, not on anyone else's who uses the same app - the only answer to that is contacting the outfit who do the flagging, which you've tried (but if they have some other contact method that might have more impact).
It's possible that what's happened is that some spam caller has "spoofed" your number, i.e. used relatively simple tricks to make their number appear as yours. If that is what happened it's nothing to do with you, and likely that your number was just chosen at random, but if many people report receiving spam calls from what appears to be your number this is what will happen. Will one "not spam" report offset this? I don't know. Will the flag eventually go away after the spammers move on to another number? I also don't know. But then I don't even know whether this is what has happened, it's just a plausible scenario.