Quickest In-Game Method (All Consoles)
The fastest way is through your Universal Profile from the Main Menu:
Make sure you are on the Main Menu, not inside any sub-mode like Diamond Dynasty or RTTS.
Select your Universal Profile icon in the top-left corner of the screen.
Go to the My Player or Settings tab within your profile.
Locate the Crossplay toggle and switch it to Off.
This method works for both PlayStation and Xbox users, but sometimes the toggle may be greyed out. If that happens, move on to the system-level method.
System-Level Methods (If In-Game Toggle is Locked)
Xbox Series X|S & Xbox One
On Xbox, controlling crossplay through the console is often more reliable (https://www.u4n.com/news/how-to-turn-off-crossplay-in-mlb-the-show-26-xboxps.html):
Press the Xbox button to open the guide.
Navigate to Settings > Account > Privacy & online safety.
Select Xbox privacy > View details & customize > Communication & multiplayer.
Find "You can join cross-network play" and set it to Block.
This ensures you will only match with other Xbox users.
PlayStation 5
PlayStation usually lets you disable crossplay in-game, but if it's locked:
Return to the Main Menu, making sure you are outside Diamond Dynasty or any other sub-mode.
Click the Gear icon (Settings) or your Profile icon.
Locate the Crossplay toggle and switch it Off.
Make sure Console Sharing and online permissions are managed, as certain restrictions can affect the toggle.
Note: Disabling crossplay may increase matchmaking wait times because the game will only search for opponents on your own console platform. If you don't mind slightly longer queues, this is the best way to play strictly with your friends or fellow console users.