Spam (or just email) filters in iOS Mail.app

This is definitely something they should have implemented by now since large umbers of people are now using their smart phones as their primary email reader.

Currently, this is how I'd do it:

1. In iOS Mail tap on the Search field at top of screen
2. Type [SPAM] in the search field
3. The screen will show results under various headings
4. Swipe up until you see the Subjects heading
5. Now tap on the Subject contains: [SPAM] option
6. This will bring up all results matching that criteria
7. Now tap on the blue Edit option at bottom right of the screen
8. Select all emails
9. Tap on Mark > Move to Junk

However, you may be able to achieve a more automated approach by creating a Shortcut with the Shortcuts app. I'm heading out now for a most of the day, but if I get a chance I'll try and create a shortcut and edit my answer to explain how I created it.

In the meantime, if you'd like to explore this as an option and you don't already have the app installed, you can download Shortcuts from the App Store.