I think a little added friction with a confirmation pop-up is perhaps more proportionate than going straight in with a full-on official warning. 
Though if certain members routinely ignore the pop-up you may still need to reach out to find out what their thinking is. They may just need a friendly nudge to realise what’s expected. 
I think if you’re looking to build your own theme component for this you may be better slipping this over to the Dev category to get more eyes on it. 
I haven’t checked to see if the necro warning still shows for me, but generally the Just In Time (JIT) messages don’t show up on mobile (which is unfortunate 
). And it does look like the Warn admins/tl4 when replying to closed topics theme component is marked as broken now.
Dev is not my forte, so I’m afraid I’m not going to be a world of help, but you can find the official guides in Documentation > Developer Guides.
Hopefully David fixes Warn admins/tl4 when replying to closed topics and then you can see what needs changing in your own one.