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Three places to fix it. Add one line to the bottom of every editing prompt, put the same line in ChatGPT's custom instructions so it applies to every conversation, or build a small Custom GPT with the rule baked in. The first is the cheapest, the third is the most reliable, and none of them is perfect: the model has been trained on millions of pages that use em dashes, and one will still slip through occasionally.

Why this question matters

You are right that em dashes are correct in professional writing. The problem is perception. A lot of readers now read an em dash as the AI's signature, the way they once read "delve into" or "in conclusion". If your writing is being judged on whether it looks AI-written, the dashes are worth removing.

How I'd approach it

The one-line fix. At the end of every editing prompt, paste: Do not use em dashes anywhere in the output. Use commas, full stops, parentheses, or colons instead. Hyphens in compound words are fine. The last sentence stops it overcorrecting and stripping the hyphen out of "well-written" or "AI-generated".

The permanent fix. Open ChatGPT, go to Settings, then Personalization, then Custom Instructions. In the box that asks how you want ChatGPT to respond, paste the same sentence. From then on, every new chat starts with that rule in place. Claude and Gemini have the equivalent setting in their own preferences pages, and the same wording works in both.

The reliable fix. If editing is a regular job rather than a one-off, build a Custom GPT. Name it "My Editor", paste your usual editing brief into the instructions box, and put the no-em-dashes rule near the top. You open that GPT instead of plain ChatGPT and the rule is already there. Claude Projects does the same job in Claude, with the bonus that you can load in a few examples of your own writing to tighten the editing pass on more than just punctuation.

The honest catch. Even with all three in place, a dash will leak through on long edits. A final pass with your text editor's find-and-replace closes the gap: search for the em dash character, replace each one with a comma, full stop, or pair of brackets depending on the sentence. The same trick catches the en dash, which some models default to as a workaround once you have banned the em dash.

What I'd avoid

I would avoid asking ChatGPT to "write more like a human". Specific instructions outperform vague ones. "Do not use em dashes" is specific. "Sound human" is not, and the model's guess at what that means may not match yours.