It is easy to do this by creating htaccess file as mentioned in the above post.
I have another idea. Redirect all the subdomain to one domain via DH panel --> Domains --> Manage Domains Create a temporary index file in the one domain which tells visitors that the site is under maintanence.
You can show visitors some message at least.
I did not try it before. You may want to try with one of your subdomain first.
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