Go into Google+ on your computer and delete the pictures. Go into Google+, which is connected with Picasa, and turn off auto back up for pictures. Make sure the synching for the Picasa albums is still off, then go into Settings > Application manager. Select the All tab, find Gallery and hit the Clear data button. Next time you open the Gallery, it will take a moment to rebuild it but the Picasa albums should finally be gone. You shouldn't even have to back up the folders first. It just wipes the thumbnails that were previously cached when the Picasa folders originally synchronized... That's why it takes a moment to rebuild afterwards.