使用している GUI フレームワークを指定していませんが、llista.curselection()を見ると、Tkinter を使用していると思われます。
以下に完全な例を示します。
from Tkinter import *
def on_delete_click():
# Get the indices selected, make them integers and create a set out of it
indices_selected = set([int(i) for i in lb.curselection()])
# Create a set with all indices present in the listbox
all_indices = set(range(lb.size()))
# Use the set difference() function to get the indices NOT selected
indices_to_del = all_indices.difference(indices_selected)
# Important to reverse the selection to delete so that the individual
# deletes in the loop takes place from the end of the list.
# Else you have this problem:
# - Example scenario: Indices to delete are list entries 0 and 1
# - First loop iteration: entry 0 gets deleted BUT now the entry at 1
# becomes 0, and entry at 2 becomes 1, etc
# - Second loop iteration: entry 1 gets deleted BUT this is actually
# theh original entry 2
# - Result is the original list entries 0 and 2 got deleted instead of 0
# and 1
indices_to_del = list(indices_to_del)[::-1]
for idx in indices_to_del:
lb.delete(idx)
if __name__ == "__main__":
gui = Tk("")
lb = Listbox(gui, selectmode="extended") # <Ctrl>+click to multi select
lb.insert(0, 'Apple')
lb.insert(1, 'Pear')
lb.insert(2, 'Bananna')
lb.insert(3, 'Guava')
lb.insert(4, 'Orange')
lb.pack()
btn_del = Button(gui, text="Delete unselected", command=on_delete_click)
btn_del.pack()
gui.mainloop()