The answer was to use the AutoFilter option on the Protect command (I also need to allow FormatCells and InsertColumns for another requirement).

exWS.Columns().AdjustToContents();
exWS.RangeUsed().SetAutoFilter();
exWS.Protect().AllowElement(XLSheetProtectionElements.FormatCells)
              .AllowElement(XLSheetProtectionElements.InsertColumns)
              .AllowElement(XLSheetProtectionElements.AutoFilter);
exWS.Column("B").Style.Protection.SetLocked(false);