You could also use MSConfig, if you don’t want to download a third party tool to do something already built into Windows.
You can use EasyBCD which has the ability to control the bootloader and with it you can delete the bootloader entry.
msconfig (Windows key + R). In the Boot tab, all operating systems will be shown. Remove the other operating system, and set Windows 7 as your default OS. See the image below for a screenshot of the Boot tab (note that I have only one OS on this machine).