Deleting a list takes it off this session. The people who were in it stay on your People pages and on any other sessions they're attached to — nobody is deleted from your app.
To delete a list:
In the schedule item, click the ⋯ button in the list's header.
Choose Delete list.
Confirm. If the list has people in it, Yapp tells you how many before you go ahead.
If you only want to take one person off a session, remove that person instead of deleting the list. There are two ⋯ menus on a schedule item: the one next to a person acts on that person, and the one in a list's header acts on the whole list.
You don't have to delete a list to hide it from attendees — a list with nobody in it doesn't show up in the app.
Related articles that might be helpful:
Start with the guide to lists on schedule items
See how you can remove one person from a schedule item