You can turn off Sharing for your Collections at any time!

To do this, open the Collection in your Client Gallery Dashboard, click on the Collection Settings tab, then scroll down to Social Sharing Buttons. Toggling the social sharing buttons to 'Off' will remove any built-in Collection and individual photo sharing functions, such as Facebook, Twitter, and Pinterest.

Note: this will also hide the Favorites sharing options, and your clients will only see the "Send to Photographer" option.