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)