I think Adobe has indicated the disappearance of the negative prompt section was not intended. So, hopefully it will return.
In the interim, you will have to add words to your prompt to try to accomplish this. When you want to include "negative" prompts in the main prompt, consider this: the model does not seem to understand the coupling of two words where one word negates the other. Because of that, you end up with the thing you are trying to eliminate actually getting added. For example,
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