It is always possible for someone to save what is shown in the browser to a hard disk (by right-clicking, by extracting it from the browser cache, by making a screenshot, etc.). It's not possible to protect the content of your online albums from being downloaded or copied. The only way to stop people from downloading your photos is by not uploading photos to the internet in the first place.
Google Photos currently does not have a setting to disable downloading of photos, which means that the photos can be downloaded in full resolution. There are some ways to limit the usability of your photos:
Add a watermark (with your name, for instance) before uploading. Google Photos has no watermark tool.
Upload copies of photos at a smaller resolution that would be sufficient for web usage, but not sufficient for printing