Cranking up one of the content tags in the negative prompt can create nicely stylized images.
I was creating Anime images using the Pony checkpoint, so I had source tags like "(source_cartoon:1)" in the negative. Somehow, it got bumped up to a higher value -- and the images suddenly became starkly stylized. They became simplified towards larger geometric shapes with uniform color fill and strong outlines.
I researched this a bit further, mostly focusing on that tag. Links below. My findings:
The amount for best effect depends on the prompt; more complex prompts require higher strengths. Values between 2 and 3 usually create the best results, but you can sometimes get interesting effects even at crazy high values.
If you want something that isn't too abstract, best start with a prompt that already creates a sharp image, and then see what happens when you crank up the negative prompt.
Note that the civitai image generator adds its own, hidden tags to the negative, so the results are weaker. Disabling the NSFW switch fixes that, but cranking up to a higher value might also work. I did only little testing with that on this site, so I can't say for sure.
Here are some examples, incl. generation data:
https://civitai.com/posts/2330812
And for research, here are comparisons for different negative prompt strength effects:
https://civitai.com/posts/2330426
Tip: In Automatic1111, you can use the X/Y/Z-plot script with prompt S/R for determining the best amount:
