Excel automatically opens files that are stored in both the Excel Startup folder and an optional alternate startup folder.
To prevent a file from opening each time you open Excel, move the workbook from either the
/Applications/Microsoft Office 2011/Office/Startup/Excel folder or the alternate startup folder to a different folder.
Tip: To determine if you are using an alternate startup folder, on the Excel menu, click Preferences, and then under Authoring, click General. If you have entered a folder in the At startup, open all files in box, this is your alternate startup folder.