Assuming that you're talking about an iCloud calendar, those alerts are being placed there because of a setting you have on your iPhone, iPad or Mac. It's a default that places alarms on all new events.

On your iPhone or iPad

Go to Settings

Tap on Mail, Contacts, Calendars

Under Calendars tap on Default Alert Times

Make sure Events and All-Day Events are set to None

On your Mac

Open iCal

Under Calendar click on Preferences

Do the following for all Selectable Accounts

Make sure Events and All Day Events are set to None

You can also manually open each trip in iCal and turn off the notification.