Unsharing a Workbook

I have a shared workbook on our server and am trying to remove data validation from a column. The workbook must be removed from Shared status to do this, however the "Allow changes by more than one user..." check box (which is checked) is grayed out. As a result I am unable to remove the workbook from shared status.

Any ideas of what could cause this or how to unshare the workbook?

Any assistance is greatly appreciated.