Hi Richard, I'm now at my PC and can give you a quick overview of how to turn off the "Buy Photo" option.
First sign in to your site.
At the top left (once you've signed in) there should be a row of buttons that start with the "SmugMug" Logo then Upload, Photos, Organise buttons etc.
Step #1.
Choose "Organise" this will then show a folder type view.
Step #2
Choose a gallery (highlight it, just click on it but don't open it) that you want to change to remove the selling option.
Step#3
You should see a "Settings" button at the left above your galleries thumbnails. Click the settings button it then opens a separate window showing all of the editable elements of the gallery.
Step#4
Choose the "Shopping" element and you will see at the top of the page a option to turn off the ability for people to be able to purchase an image. Once you turn this off and pressed "SAVE" a visitor to your website will not see a "buy photo" button. However as I said in my previous reply that Buy Photo button will still be visible when you are signed in to your website.
To verify that you have removed the buy option sign out of the website, view the gallery that you just edited and the "buy photo" should not be visible. If that worked as expected you can then repeat the process for each gallery.
I hope this helps.