I would just add an option to not include a user, or users from the search results.
Of course, if you search by user - you can not exclude a user at the same time.
I would probably approach this by giving an option to either search by user(s) / or excluding user(s)
So one input field, with one dropdown selection to specify is this about including or excluding, and if the field is empty the dropdown is dismissed.
Delimiter - Good question, can't say really off the top of my head.