Q:

How to filter unwanted packages in android SDK? 

Android SDK updater shows a lot of different system images.  I don't need them all.  Is there an option to check which packages I am interested to download in Android Studio

However, I cannot uncheck any system position image (only Android Repository and Offline Repo can be unchecked). 

A:

I think its important to clarify couple of things.  Lets take your screenshot as base:
SDK platforms: It includes all android API.  if any SDK platform installed .then it will be part of update-tracking. 
SDK Tools: It includes developer tools.  any checked one will be apart update-tracking. 
SDK Update sites: It includes repositories for all resources. 
If you don't want to get any update of any system image.. just don't install any image.  Then You won't be notified about updates.  Im pretty sure.  You will get updates about what you have installed only. 
If you still interested to uncheck Android System Images, do the following:

Launch SDK manager from SDK platforms by click on Launch
standalone SDK manager
Go to menu under Tools and click on Android Add-on Sites...
Uncheck Android System Images
Close and that's it 

If you cannot click on Tools in menu.  Try to change focus to another window and get back to SDK Manager Window.  It happens with me on Mac

Good luck,'.