Hi Ingo,

Only by using Custom CSS. In Template Options > Custom Code > Custom CSS or use different area if you have not Helix-based template.

.speasyimagegallery-gallery-item, 
.speasyimagegallery-gallery-item:hover  {cursor: default; pointer-events: none;}

in Desktop/Tablet/Smartphone view is should disable Zoom and all typical actions for those thumbnails.