There are many ways to blur people's faces so that they are not recognisable. The easiest way to do this, perhaps, is using Gimp's "Smudge" tool. Gimp is an open source image manipulation programme and can be downloaded here
Open your image with Gimp.
Now the image is loaded and appears in a seperate window.
Choose the "Smudge" tool in the tool panel.
Apply the smudging by pointing to the face and pressing the left mouse click until the face is blurred and unrecognizable. Do this at least twice in different directions for each spot you're blurring so that it becomes difficult to reverse your edit.
exif thumbnails do not get updated automatically when changing the image in Gimp. There is, however, an exif thumbnail editor contained in Gimp 2.2-pre2 and greater. "exiv2 -v rm *.jpg" can be used to remove all exif/iptc data off all jpg files in the current directory. The exiv2 command is a package available for Debian here.