On submitting an answer, the system could check if at least 75% (more or less) of the answer exactly matches either the Markdown code or the rendered text of the question or another answer, and reject such an answer with a message similar to the following:
###Oops! Your answer couldn't be submitted because:
- We're sorry, but as a spam prevention mechanism, we cannot accept answers with content mostly copied from (the question | another answer). Please add more content of your own.
As a side note, I will no longer add spam warnings to answers. I will add a comment to the same effect instead.