The disable solved education message setting controls whether this message shows up in the composer or not when you try and add another post to a topic which already has a marked solution:
### This topic has been solved
Only reply here if:
- You have additional details
- The solution doesn't work for you
If you have an unrelated issue, please [start a new topic](%{base_url}/new-topic) instead.
You can customise it to be whatever you like by going to your /admin/customize/site_texts page and either searching for education.topic_is_solved, or a snippet of the text. 
There’s also another little Solved message which isn’t affected by that setting which encourages the OP to mark a solution:
That can be tweaked by looking for these in the same place as the other:
js.solved.no_answer.title
Has your question been answered?
js.solved.no_answer.description
Highlight the answer and help others by using the solution button below the correct reply.
(Though there is an open feature request New "Has your question been answered" popup can't be permanently dismissed to add a setting to this one too)