Step 1: Create a new field in your database called 'Excluded?' or similar, using the 'Edit Custom Fields' page. Add 'Yes' to this column for any member you don't want to be on your main mailing list. (You can also add this column to a CSV file of new member data and upload the file using 'Add Members').
Step 2: Create a new list on the 'Member Lists' page called 'Not Excluded' or similar. Set this list to include members by a filter on the 'List Members' tab. The filter should read 'Excluded? > doesn't contain > Yes'
Step 3: This list will now automatically update every time you add a new member. Members added to the group normally will be on the 'Not Excluded' list. Any member added or edited to include 'Yes' in the 'Excluded?' column will be in the main database (therefore the default 'All Members' list).
Tip: It's also worth creating a new list with a filter 'Excluded? > contains > Yes' and calling this list 'Excluded members' or similar, then all members will be on one list or the other.