A very interesting topic discussed in Working in Public - Nadia Eghbal is that there may be contributions to open source projects that are extractive. This means that the time and resources needed to deal with the contribution have a higher value than the one derived from the contribution itself.
This is what hacktoberfest has made evident:
And each of them requires maintainer time to visit the pull request page, evaluate its spamminess, close it, tag it as spam, lock the thread to prevent further spam comments, and then report the spammer to GitHub in the hopes of stopping their time-wasting rampage.
If you try to incentivize contributions, there is no way you can have control on the type of help you will get.
I don't see a clear path out of this besides limiting contributions to a group of people in which developers can place trust. However, a contribution may be an issue, and I don't think those should be limited.
Also, an extractive contribution may be only the first contribution of a new developer, and therefore spending the time to introduce them to the inner workings of the project may pay off in the long run.
The question is, therefore, how do we judge the value of the contribution and the value added/extracted (see: the value of code). If the project is somehow seen as a community empowering endeavor, being receptive of new contributions may be required (like having a teacher answering questions instead of dismissing them). But not all projects need to be like this.