How to wipe an SSD before doing a clean install?

I have an older netbook that had been running XP Home for years. Last week I installed Windows 10 Pro on it, but it ran terribly slow. Now I'd like to try installing Windows 7 Pro.

When I installed Windows 10 I also replaced the system HDD with an SSD, so it was unformatted when I began. I made a USB flash key for the installation and it booted and installed Windows 10 without any problems.

I created a Windows 7 flash key the same way (using the Win7 DVD/USB tool and a downloaded ISO), but was unable to get the system to boot from the flash key instead of the system disk. No matter what I set in the BIOS, it kept booting into Windows 10 from the SSD. So I ran setup.exe from Within Windows 10 and it installed Windows 7 OK, but I see that there's a windows.old folder and a couple other old folders in the root of C:.

I'd prefer to start over again and wipe the SSD clean before installing Win 7. Maybe the normal setup when booting from the flash key allows you to reformat the drive, but if I can't get the system to boot from the flash key, that's a non-starter.

Is there a simple method that I can use, short of pulling the SSD and reformatting it in another machine?

As your netbook is old, your bios must be Legacy and you need to install the Win 7 iso on the USB as Legacy - MBR.
As AddRam said, all you need is to boot the Win 7 USB installation disk, choose install - Custom - Advanced - delete all partitions and then install.