Remove members: open the list, tick the checkbox in front of each member (or the header checkbox to select the whole page) and use the remove action that appears; confirm Are you sure you want to remove N contacts?. This only removes them from the list – the contact or customer record itself is kept.

Delete a list: open the list, click the pencil icon next to its name (or Edit in the Lists table), then Delete and confirm Are you sure you want to delete this list? Your data will be lost. The list is deactivated and disappears from the Lists page and the recipient picker. System lists such as Email Newsletter have no Edit/Delete option.