How do you clear .thumbnails folder without hanging Windows Explorer?
You could use any file explorer directly on your Android device to do this. A good choice is e.g. ES File Explorer File Manager, which as a side-effect also enables you to directly access your PC from the Android device (accessing your Shared Folders via WiFi), and has many other nice features:


Start the app
Go to Settings &rarr; File Settings, and enable Show Hidden Files option
Navigate to the folder
long-press the folder name, and select "delete" from the pop-up menu
Confirm the deletion of folder and contents, if asked for
for "normal operations", you might want to disable Show Hidden Files again


It is pretty safe to delete the entire folder (instead of deleting all files separately). After all, with a new phone/sd card, the folder won't exist either. The app/service using the folder will simply re-create it if it does not (longer) exist.