Well, perhaps you're using the wrong tool - most of the assets for Daz Studio are specifically
designed to render in a photorealistic manner.
However, there are a few things you can try:
- Use either the Filament or the 3Delight renderer instead of iRay. These are less photorealistic. They're also a good bit faster.
- Modify your assets. One of the reasons that the iRay renders are more photorealistic is that the assets tend to have very detailed textures and shaders associated with them. Turn off things like normal maps and bump maps. Turn off Sub-Surface Scattering (SSS). And possibly other changes. These will make the skins look "flatter" and more "plastic."
- In an extreme case, there are specialized add-ons to Daz that will give the results a more "cartoony" look. You may be able to use these "not to their full extent" to flatten out the colors somewhat.