The workbook sharing feature in Excel 2013 enables multiple parties to collaborate on a single document. Although it's a handy feature, it's not for everyone so Microsoft has made it easy to disable. Before you can turn sharing off completely in a workbook, you must first remove all users from the list of people currently authorized to collaborate on the document.
Click "Share Workbook" in the Changes group on the Review tab in Excel and review the list of names on the Who Has This Workbook Open now list. This list is located on the Editing tab and it contains the list of all people currently authorized to share the workbook.
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Select a user you want to remove from the list and click "Remove User." Repeat this step for all of the users on the list other than yourself. You must remove all active users from a workbook before you can turn sharing off all together.
Click "Share Workbook" in the Changes group on the Review tab and then verify that your name is the only one listed on the Editing tab.
Remove the check mark from the "Allow changes by more than one user at the same time" checkbox to stop sharing the workbook.
If this checkbox is unavailable, you must remove protection from the workbook. Click "OK" on the Share Workbook dialog to close it and then click "Unprotect Shared Workbook" in the Changes group on the Review tab. Enter the workbook password, if prompted, and click "OK." You can now remove the check mark from the "Allow changes" checkbox.