There are two ways to unsubscribe from receiving push messages from the game.

[Game Settings]
If you only want selected type of push messages, you can set it up through the game settings.
Game Settings (Cogwheel) > Push Notifications > Push Settings > Turn off messages for selected types of messages

[Device Settings]
If you do not want to receive any type of push messages from the game, you can turn them off entirely through your device settings.
Go to your device and/or app settings and turn off all notifications from Homerun Clash.
Doing so will disable all messages from the game even if you have them 'ON' in the game settings.