You'd have to find a way to opt out of copy and paste. Even if you could do that someone could take a screenshot (or a photo of their screen) and use the image as input.

What's your concern here - is it not wanting LLMs to train future models on your content, or a more general dislike of the technology as a whole?

The "not train on my content" thing is unfortunately complicated. OpenAI and Anthropic don't train on content sent to their APIs but some other providers do under certain circumstances - Gemini in particular use data sent to their free tier "to improve our products" but not data sent to their paid tiers.

This has the weird result that it's rude to copy and paste other people's content into some LLMs but not others!

I've not seen anyone explicitly say "please don't share my content with LLMs that train on their input" because almost nobody will have the LLM literacy to follow that instruction!