You can search for/filter your members based on a number of criteria in the Member Management area of the Admin Control Panel... and one of those filters is for "Last Visit Date", so you can define inactivity on your terms in that regard. There is no way to see how many/which members have suspended all email notifications currently however.
When you block a person, they can no longer invite you to a private message or post to your profile wall. Replies and comments they make will be collapsed/hidden by default. Finally, you'll never receive email notifications about content they create or likes they designate for your content.
Note: if you proceed, you will no longer be following .