Is there a simple way to prevent GalSim from shooting all the photons from a star into a single pixel?
Stars are inherently supposed to look like point sources if you don't have any PSF at all (no atmosphere, no diffraction).  They are a delta function in that case, so all of the photons should fall into a single pixel.  GalSim is doing exactly what you are asking it to do.
It sounds like you actually do want to have a PSF; I suggest using the galsim.Airy class, representing a diffraction-limited PSF.
I'm generating PSF-free images, so no atmosphere and no diffraction, and the images I'm getting out have stars in "quantized" positions. I'm wondering if there is an option in GalSim to prevent this, i.e. to have a more sinc-like distribution of the photons, so the behaviour of photons landing somewhere between pixels is taken into account. If there isn't an option for this, I suppose I would need to create my own sinc-function PSF and implement it around the drawImage() step?