Sample Code:

//Get all scheduled jobs
List<CronTrigger> ctList = [SELECT Id, CreatedDate, CronExpression, CronJobDetailId, EndTime,
                            NextFireTime, OwnerId, PreviousFireTime, StartTime, State,
                            TimesTriggered FROM CronTrigger];
//Abort all jobs one by one
for(CronTrigger ct:ctList){
    System.abortJob(ct.Id);
}