My Yahoo Calendar is filled with a dai...

There are a couple of ways.
1) in Yahoo Calendar for that event, you should see "Delete". If you click on that, you can then choose "All dates", "All future dates" or "this date only". Click on "All future dates", then click on <Delete>

OR 2) in the area for the event, there should be the text "Repeating event". Click on that. By default, the "Apply changes to" will be set to "All dates". Scroll down to the "Repeating" section. Select "This event does not repeat". Then click on the <Update> button