Tap on the button in the bottom right of the screen with four boxes. Then tap on the 'Spam Filter' button to show the spam rules. You can then turn these rules on or off and also edit their settings.

You can change the spam settings to move spam emails to the spam folder. You can also change the spam detector level and unsubscribe action in the 'Spam Detector' email rule.
You can active the 'Simple Spam Block List' rule to delete emails from email addresses that you have blocked.

You can change the spam rule to move suspected spam emails to the spam folder. Open the 'Spam' rule to edit it. Then turn on the 'Move to folder Spam' button. The spam score value changes how sensitive the spam rule is. (More Details)