I had the exact same problem in Excel 2015 for quite a while, and when I hid the Personal.xlsb workbook, a blank page would open instead and I couldn't close that one without closing the file I actually wanted open.
But I finally found a solution! My problem was that I had a Personal.xlsb file in an alternate location, and for some reason a search of the C drive wasn't returning a match for that file name. My location was the same one that Amber mentioned above, but I wanted to share how I found it in case someone else has a different location.
The solution is to find the real location of the Personal.xlsb file so you can delete it. To do that, open excel and switch to the Personal.xlsb file. Now select the "File" tab on the top left which will load an "Info" page. On the right side of the information on that page there is information such as "Properties, Related Dates, Related People, and Related Documents." Directly under Related Documents is a hyperlink that says "Open File Location." Click this hyperlink and excel will open the true location of the Personal.xlsb file regardless of where your particular installation has placed it.
Now close Excel, delete the Personal.xlsb file, and reopen Excel. It should open only the file you want and a second window will no longer appear. From this point on, I expect other tips online about creating/deleting/showing/hiding the Personal.xlsb file will work as expected. But I have left it deleted and Excel is finally loading as I expect.