You need to use sp_update_alert and set the enabled value to 0
USE [msdb]
GO
EXEC msdb.dbo.sp_update_alert @name=N'Respond to DEADLOCK_GRAPH',
@message_id=0,
@severity=0,
@enabled=0,
@delay_between_responses=0,
@include_event_description_in=5,
@database_name=N'',
@notification_message=N'',
@event_description_keyword=N'',
@performance_condition=N'',
@wmi_namespace=N'\\.\root\Microsoft\SqlServer\ServerEvents\MSSQLSERVER',
@wmi_query=N'SELECT * FROM DEADLOCK_GRAPH',
@job_id=N'23196807-e886-4d50-9407-295b32f68105'
GO
Abhay Chaudhary
Sr.DBA (MCITP/MCTS :SQL Server 2005/2008 ,OCP 9i)