3) As an extension of option 2, you could set up a new site that is just a "splash page" that temporarily uses the domain of your original site, while running the original site on a temporary domain.
a) In the original site, go to Site > Settings > Domains tab. Add a new domain that you won't want to use with any other site (for example: tempclosed.yourdomain.com). Then, make that new domain the primary domain. Finally, delete the original domain and click Save.
b) Duplicate the original site (this may be the easiest option). Give the duplicate the original domain but UNCHECK all the options of items to be copied, especially the users, since you don't actually want to have any users in the new site. Please note: You will want to make this new site a Customer-specific (B2B) site when you duplicate it, as all plans include unlimited B2B sites.
c) In the new site, go to Site > Settings > Instructions tab. In the "Login Page Instructions" area, add the text and/or images that you want to use to convey that the site is temporarily not taking orders. Click Save when you are done.
d) Once the changes go through in our server (usually 1/2 hour or less), when your client goes to visit the original domain, they'll be directed to the new login page with your message on it. If they try to log in, they will not be able to, since they don't exist as a user in this new site.
e) Once you're ready to take the original site online again, go to Site > Settings > Domains tab in the duplicate site. Add a new domain that you won't want to use on any other site. Then, make that new domain the primary domain. Finally, delete the original domain and click Save. Then, go to Site > Settings > Domains tab in the original site. Add the original domain back to that site and make it the primary domain. Delete the temporary domain, and then click Save. Once all this is done, you can delete your temporary site if desired.