People have even put up status update warning friends taking exterme measures to ban people sending such unwanted invites.
So here is a way to still be friends and not get annoyed by such invites on Facebook.
You can actually block such apps by going to Facebook settings by clicking on top right little triangle and then going to 'Blocking' tab on left pane and type the name and select the app that you want to block.
You can also 'Turn Off' notifications by hovering your mouse on the app or group notification and clicking on the 'X" you will get a option to 'Turn Off.
You can also change the email notification settings on the 'notification' Facebook page to free your cluttered email account.
This has worked for me for past few years. Settings - Blocking - Block apps.
