How to block image only spam in outlook?

If the messages are already filtered out automatically, there isn't much that you can do to permanently delete them directly via a rule. This is because rules work after the Junk Email filter to prevent them from still landing in your regular folders.

You can however run a rule manually to still have them removed permanently before you further analyze your Junk Email folder for false-positives.

For step-by-step instructions to do this see; Applying message rules to the Junk E-mail folder - Robert 365

Your suggested alternative of starting over entirely with a new mailbox isn't needed. If you want to start over with a new email address, you can add a new alias to your existing Microsoft Account. You could remove the old one, but be aware that this address will then be gone for good. The contents of your mailbox and everything else linked to your Microsoft Account will remain.

The links below contain more info about the difference between closing/deleting your account and creating a new one, and adding and removing aliases (email addresses) to your existing account.

See; Close your Outlook.com account - Microsoft Support
And; Add or remove an email alias in Outlook.com - Microsoft Support