Yeah. Best approach I can think of would be something like:
Cool, looks like you've getting the hang of creating Pull
Requests on GitHub. :)
The actual change here though isn't useful to us. :(
Would you be ok spending some time improving [XYZ] instead? :)
eg combine encouragement for the bit they got right + info on what needs work, and point them in the right direction for fixing it
Some people will probably just not be bothered, but others might get involved in the suggested way. Hopefully. :)
The problem is that this takes time. On one PR, sure, okay. But if you have 50 of the spammy PRs, even with Copy and Paste, this will take several minutes (maybe up to an hour, depending) of a maintainers day (and really, per day, however long Hacktoberfest goes on). That's time that they're not spending coding or updating docs in a valid way or spending with family. It would probably be tenable if there were some heuristic that was 100% certain that this was a spammy PR, but I'm pretty sure that's not the case.
Some people will probably just not be bothered, but others might get involved in the suggested way. Hopefully. :)