Assumes your docker images list looks like:
<none> <none> 2b07576f6f81 ...
What you need from this list is 2b07576f6f81, which is the ID.
PS C:\> docker images | Select-String -Pattern '<none>' | foreach-object {docker rmi $_.tostring().split()[28]}
Comments welcome, especially if you know how to achieve a consistent split of docker images
output that accounts for named images of varying length.