To delete a Calendar item, you first have to select the item you want to delete.

If you are viewing the Calendar in the Day view, after selecting the specific item you want to delete, click on the menu  Edit    Delete  

If you are viewing the Calendar as Week or Month, click on the item's action button and choose Delete.

If you are viewing the Calendar as Year, you cannot delete itens.

Note:
When you delete any object in BSCW, it goes first to your WasteBasket. Thus, if somehow you have deleted the wrong item of your calendar, you can undelete it, using this function on your WasteBasket.