Edit: Thanks for your kind comments on disabling contributions. I'm aware of the possibility, this is just a meme.
Contributions in open source projects are great, needed and make the world a better place after all. Therefore they should be declined politely, optionally by giving a reason.
I hereby inform you that I will no longer accept contributions to this post /s.
I'd probably reject PRs into my FOSS projects, because the aim is to put the code out there for other people to see and do what they want with it. I'm expressing myself, and sharing what I made; I'm not setting up a tentpole project for others to pitch in and take over on, and start managing their contributions and collaborate with them.
If they want to take the code and do something with it, great. I don't want to be involved in their endeavour.