Hi there! It sounds like Excel's AutoRecover feature is turned on, which automatically saves a backup copy of your file every few minutes. This is a useful feature in case Excel crashes or your computer shuts down unexpectedly, but it can also be a bit annoying if you don't want all those backup files cluttering up your folders.

Here's how you can turn off the AutoRecover feature in Excel:

1. Open Excel and click on the "File" tab in the ribbon at the top of the screen.
2. Click on "Options" in the left-hand menu.
3. In the Excel Options dialog box, click on "Save" in the left-hand menu.
4. Under "Save workbooks", you should see a checkbox labeled "Save AutoRecover information every X minutes". Uncheck this box.
5. Click "OK" to save your changes and close the Excel Options dialog box.

That's it! Excel should no longer save backup files with the "Backup of" prefix and the .xlk extension. If you want to turn the AutoRecover feature back on in the future, just follow the same steps and check the "Save AutoRecover information every X minutes" box again.

I hope that helps!