Can I toggle the visibility of the samples in my portfolio?

No. You can either choose to enter samples which will be publicly visible to visitors to your profile or choose not to enter these samples. For the latter option you can tick the box next to "I choose not to enter any sample translations at this time" in the "Portfolio/Sample Translations" in the "Professional history" section of your Profile Updater.

Another option would be to enter samples and removing the sensitive information you would not like to be disclosed. Make sure you are respecting confidentiality agreements when entering these samples.