Gallery settings > nextgen basic slideshow > show thumbnail link > No
for general settings
Attach to post > nextgen basic slideshow > customize the display settings > show thumbnail link > No
for individual setting in post/page editor
@tizz: thanks again for jumping in to help others. You’re a hero.
@ferryverhoeve: just to add a bit here, once you’ve turned off those options as Tizz has suggested, you just need to be sure you add the gallery as a slideshow. If you do that, along with the settings above, you’ll be displaying a slideshow with no option to convert it to thumbnails. Thanks! (Erick)
As said above, ATP is in post/page visual editor, Nextgen button.
This is for putting the gallery/slideshow on the page, and for adjust individual settings.
For the general settings, you have to set Gallery settings/Other options in the Nextgen menu on the left.