“Does nothing” may have just silently unregistered it. After doing both of those, try deleting/renaming the DLL, then rebooting, and see if it’s still there. (Some portions of install/uninstall changes require reboots, at least on some machines; it may be that on your machine, you need to reboot before it leaves your context menu.)
Also, check the registry (use regedit), and see if there’s anything in HKEY_CLASSES_ROOT\CLSID that contains NppShell.dll (when mine is registered, I find it under the right {…} in the InProcServer32 subkey, and then elsewhere under the HKEY_CLASSES_ROOT\PackagedCom\Package\NotepadPlusPlus_1.0.0.0_neutral__...\class\{...} . If you delete it from both of those, I believe it will be effectively un
If it doesn’t let you delete the DLL even using UAC permission (elevated/administrator) because it’s still “in use”, then boot into recovery mode (which I believe will not load that registered DLL), and delete the file
I wish programs would just stop cluttering your context menu with useless entries
A: not useless to everybody. And in fact, the vast majority of conversations in the Community Forum about the NppShell are when people aren’t getting the Edit with Notepad++ and desparately want it.
B: Portable versions (like the portable zip that Notepad++ distributes) are your friend, if you want to not use things like shell context menus: avoiding operating system entanglements is one of the primary reasons portable versions exist.