It sounds like you may have subscribed to the iCalendar feed for your task list. To unsubscribe, open Settings from the home screen. Tap on "Mail, Contacts, Calendars" and you should see the feed listed on the right side of the screen.

Tap on the calendar you want to remove, then tap "Delete Account" to remove it.

Hope that helps!

Thanks for answer. Can you please advice me how to do it ? I don't know in which app I should do it. You wrote "open Settings from the home screen, but in which app? Thanks

jirialfiala,
Sorry for not being clearer. You perform this step in the Settings screen of your device itself, not in any particular app. From the home screen where all your apps are displayed, tap Settings.

Hope that helps!