First assign a field name to one of the custom fields (name it “Excluded” for example by editing (in preferences) each of the the ref types that you need to mark as excluded – put your asterisk in the field of each record you want to mark. If you already have these records in a group, you can make this edit globally by using the copy/change/move tool.
Edit your output style to contain that field either in the front of the Author or at the end after the final field.
The bibliography template would then look something like the below for “journal” ref type (you could superscript the field if it looks better to you that way, and if you want a space before the author, make sure you add it before the | character and that you use a “link adjacent” character from the field dropdown.
|Exclude|Author,| Title. |Journal, |Year|. Volume|(Issue)|: p. Pages|.