You can block users on Picasa Web Albums to prevent them from interacting with you. Block User is available on your list of Followers and also next to comments on your photos. Clicking Block User does the following:
- Blocks that person from leaving any future comments.
- Removes that person from your list of Picasa Web Albums Followers and blocks them from following you in the future.
If the person you block is a Google+ user, they will also be blocked in Google+, Google Buzz, and Google Reader. Learn more about blocking people in Google+.
Block User will not remove that person from any album 'Shared with' lists. You'll need to separately remove their access.