When you don’t want a user’s stream or any interaction we used Block option to do that. Now Google Plus added “ignore” option with which you won’t see their posts in the incoming stream, won’t get informed about their activity and won’t be in your circles.
At the core the Ignore option just hides the user on Google Plus while blocking completely eliminates the interaction. The following video uploaded by Google gives a brief idea of what an “ignore” option does.
You can ignore the user in your incoming or in your notifications box or in the circles page.
The ignored users can be undone by going into more actions on your circles page.