If you just want to deactivate some shields, you can do it from Avast GUI:
Open Avast GUI -> select the unwanted shield -> press "STOP";
Then go to the general settings (upper part of the GUI) -> Status Bar -> UNcheck the shields you don't want to be monitored by the Avast Tray Icon (so to avoid the warning that the specific deactivated shield is inactive).
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Otherwise, maybe like this? :
Control Panel -> "Add/Remove Programs" -> Avast -> change -> UNselect the shields you don't want -> reboot.
That should leave Avast while it should "completely" uninstall the unwanted shields.