With --help you can see the that there is a remove option:

> microk8s.ctr -n k8s.io images --help
NAME:
   ctr images - manage images

USAGE:
   ctr images command [command options] [arguments...]

COMMANDS:
     check       check that an image has all content available locally
     export      export an image
     import      import images
     list, ls    list images known to containerd
     pull        pull an image from a remote
     push        push an image to a remote
     remove, rm  remove one or more images by reference
     label       set and clear labels for an image

OPTIONS:
   --help, -h  show help

So you can see the running containers with:

 microk8s.ctr -n k8s.io containers ls

And later remove images with:

 microk8s.ctr -n k8s.io images rm "image_ref"