Sure, the list comes from the update files you've downloaded in the past. You can find those files here:
Windows: <user home folder>\Spine\updates\
Mac: <user home folder>/Library/Application Support/Spine/updates/
Linux: <user home folder>/.spine/updates/
You can simply delete the files there for the versions you don't want to see in the list. If you want to use a version not in the list, you can click Other... in Spine and type the version you need.
