1) Go to Attendees under Conferences.
2) Select the conference you want deleted.
3) Click Uninvite protection on the top right hand corner (to make sure it is unchecked).
4) Unselect all the invited teachers (do the same with teams if you have any and with parents if the conference is private).
5) Click Save.
Once you do this for each conference you would like to delete, you can go to List conferences and remove the ones you do not need.