There are a couple ways to do this, in terms of user education:
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customize the pop-up overlay guidance that appears for your first two topics and first two replies, to mention this rule
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customize the default “type here” text that appears in the editor to mention this rule
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mention it in the “about this site” pinned topic that you fill out during the site setup wizard on initial install, and appears as the first visible “what the heck is this place” topic for all users (though pinned topics will un-pin after you’ve read them, for logged in users)
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there’s a lot of quoting functionality built in which you can potentially hide via simple
display:none;override CSS rules, such as the quote button in the editor, the quote button that pops up when you select text, etc
Are there any communities that you’re aware of which enforce this rule, either through software or through public moderator / community rules?
I definitely get how this can be way too much. Let me post an example. 
All text below this point is a PARODY. I am only assuming this style for the sake of showing a BAD EXAMPLE of what you DON’T WANT 
How interested are you? Interested enough to pay for the effort?
What do you mean by projecting?
Define “original”. Nothing is original.
Maybe for some folks but not all!
No such thing exists; there’s no way to make it ironclad.
On what platform?