This is what Hacktoberfest should have been from the start. A company incentivizing pull requests against your repo should require your consent. A maintainer needs to affirmatively sign up for this sort of work; they can't have it thrust upon them. As a maintainer, all the other half-measures people have suggested (e.g., accepted PRs only; account age measurements; filtering out README changes) do not actually reduce the unasked-for burden on me.
That said, I'm not sure this year can be salvaged. It's not clear whether all the existing spammers will get the message. I hope DigitalOcean emails all folks signed up to notify them of this change. (They haven't communicated any of their changes so far in such a way.) Notably, a day ago, the DigitalOcean staff thought this wouldn't make a difference. [1] And it's even possible this will just result in issue spam begging maintainers to add the Hacktoberfest topic.
But it's also possible that this will work out. The best version of this is that low-quality contributors looking for a t-shirt get directed toward honeypot repositories, and folks interested in making substantial contributions will continue using issue labels like GitHub's own "good first issue", or the Hacktoberfest repository topic, to find places to contribute.
We'll see in a day or two whether this is enough.