MagicISO is ancient ISO-image editing app. If you want to install it and see the ancient look it has and notice that the trial versio is hugely crippled, you may choose to uninstall it like I did.
Guess what! It does not remove the shell extension and the DLL-file associated with it. The software is gone and there is very little you can do to get rid of the context menu.
I found that NirSoft's ShellExView is the tool for that job. Just pick the MagicISO context menu and disable it. The menu does not disappear from the context menu automatically. Explorer needs to be reloaded for that. I simply killed the process with Windows Task Manager and restarted it.