I think a 'warning' is the most appropriate - sometimes posters (especially OPs) may want to come back to say 'this is the outcome / thank you for helping me' or similar.
I would add that the warning could include ' you may get a better response if you start a new thread'
I think some posters google their issue / problem, get led to a thread here, and sign in and post without really checking the site.
The Republic of Ireland and their tensions with migrants.
Harry and Meghan ‘royal’ tour of Nigeria.
Film Titles With Words In Common
Forgetting where you left your keys dies not mean you are “losing it”