PLease Help: Dropdown list that removes previously selected item

Hello-

I've been asked to develop a spreadsheet that will work for scheduling. I've created a drop down list of all supervisors. Due to way scheduling is done, they are placed in cells B6, C6, F6, G6, B17, C17, F17, G17, ect. Once selected on the spreadsheet for that day, I do not want them to be available as an option. So if I put Bailey in C6, I don't want to be able to put him in F17.

I'm not sure if this is possible. Any help will be greatly appreciated.

Thanks!