There are 2 ways to make photos unavailable to your clients:
Un-publish albums
If you want to hide only selective photos from your clients, simply upload un-publish the albums you don't wish your clients should see.
Closing Gallery manually
However, if you want to make the entire gallery inaccessible to your clients, simply enter that client's gallery, navigate to Gallery Settings on the left side settings column and select Close Gallery. This will make the entire gallery inaccessible to your client while you won't lose out on any data on your end.