Some margin cushion notifications are configured as alerts within TWS. To disable alerts simply log in in to TWS Mosaic and click New Window followed by Alerts and locate the alert in question to cancel it.
Margin cushion notifications might have been set up as well in Trading Assistant within IBKR Mobile. To disable them, simply log in to IBKR Mobile, tap Trading Assistant from the menu on the left and uncheck the Account Notifications.
However, when an account's Excess Liquidity as a percentage of the Net Liquidation Value falls below 5% the system automatically sends out a margin warning message. This message is sent as a courtesy for the account holder to check the account as it is their responsibility to ensure the account does not fall into an Excess Liquidity deficit. If an account has a negative excess liquidity number then the Liquidation system will automatically close positions.
For more information, please visit the IBKR Knowledge Base.