To reset your schedule, simply click on the schedule tab from the left hand menu.
Once in your schedule, above your calendar you will see a 'reset schedule' button. Use this button to reset back to the default schedule you had upon first logging in to SISOMA.
Don't forget, this will remove all posts you have manually added in to the schedule.
If you want to remove just one post or a post repeated weekly/monthly, click on this post in the calendar and click on the pencil icon in the post preview on the right. Here you can choose to delete (for just that one post), or delete all (to delete the series).