You have to code to populate the ComboBox initially (using all names from Column B).
You need the check for each entry in each project wether this entry is in your ComboBox (if Yes, delete it).

And for the event when you select a new name from the combobox for a project, you need to delete that name from the combobox.

Speaking about this ComboBox, doesn't each Project have one of them, in this case you need to do the above actions for all of them.