Hi!
There are two choices depending upon what you want to achieve.
If you just want to prevent users from participating, You could put your discourse in a read-only mode by visiting admin > backups and clicking the Enable read-only
option.
Alternatively, if you want to take your discourse down, you could follow this guide:
Once you’ve added the offline page, you can simply pass the following command: ./launcher stop app
which will stop the discourse container and perpetually display the offline page unless you restart the container.