Excerpt from https://learn.microsoft.com/en-us/office/troubleshoot/excel/files-open-automatically
Remove files from the XLStart folder and the alternative startup folders
To remove files from the XLStart folder and the alternative startup folders, follow these steps:
- Select Start, and then select Run.
- In the Open box, type one of the following and press Enter:
- For Office 365 or 2019 32 bit:C:\Program Files (x86)\Microsoft Office\root\xx\XLSTART
- For Office 365 or 2019 64 bit:C:\Program Files\Microsoft Office\root\xx\XLSTART
Where “xx” represents the version that you are using (for example, Office15, Office14, etc.).
Also try the following:
- %appdata%\Microsoft\excel\XLSTART
- C:\Users\UserName\AppData\Roaming\Microsoft\excel\XLSTART
- Delete any files in the XLStart folder or move them to another folder.
- Restart Excel.