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:


In iOS Mail tap on the Search field at top of screen
Type [SPAM] in the search field
The screen will show results under various headings
Swipe up until you see the Subjects heading
Now tap on the Subject contains: [SPAM] option
This will bring up all results matching that criteria
Now tap on the blue Edit option at bottom right of the screen
Select all emails
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.