I would even take it a step further and allow maintainers the ability to cherry pick certain issues they'd like to put up for hacktober.

I'd still keep the repo wide tag for opt-in, but also optionally provide issue level opt-in.

Alternatively, you can provide a platform where participants can self regulate...e.g. like spammers called out on r/programming or maybe a spam detection algo.

Edit: maybe the incentives can also be altered. E.g. sticker for non opt-in repos, t-shirt for opt-in ones. Hmm solving bad actor behaviour is an interesting problem