This Seeking Tech guide will show you how to disable the cross-platform matchmaking in the PlayStation 5 version of Marathon. Follow the video and text guide below:
Step 1: From the Main Menu, select the Game Menu option on the bottom right.
Step 2: Select the Settings option.
Step 3: Select the Social option on the left.
Step 4: Set the Crossplay setting to Off.
The game will then automatically sign you out before signing you back in. Crossplay should now be disabled.