Picasa imports and categorises the images and videos on your hard drive according to the folders in which they appear. In addition, Picasa scans your images for instances in which human faces appear and gives you the option to tag those images with the names of people you recognise. Over time, Picasa gets better at suggesting possible name matches for people appearing in your photos. You can then sort your photos by names of persons. If you do not recognise a person in Picasa's facial recognition feature, you can delete that instance so that it does not come up again.
Launch Picasa. Click the arrow next to the "People" category in the left navigation menu to expand it.
Click the "Unnamed" subcategory in the "People" category to display pictures of persons currently unknown in Picasa.
Click the "X" in the upper-right corner of any picture of an unknown person you want to delete. That picture disappears.
Repeat Step 3 until all unknown persons have been deleted from Picasa.
Every time you add more pictures to Picasa, more unknown persons may appear in the "Unnamed" subcategory under "People" in the left navigation menu. You can delete any of those people as necessary.