They should be in something like [ %APPDATA%/RenPy/save_directory ] for windows or [ ~/Library/RenPy/save_directory ] for mac.
Save files are different from persistent data though. I think they're held in the same place, but persistent data is what hold achievement and overall game progress.
One way to get rid of them for sure would be to duplicate and back up your whole game folder, then go into the actual game and use the big red 'delete all progress' button on the collections page. That button wipes out all save files and persistent data so you can start from scratch.