#!/usr/bin/env python3
# coding: utf-8

from PyQt5.QtWidgets import QWidget, QGridLayout, QComboBox, QApplication
import sys

class Window(QWidget):
    def __init__(self):
        QWidget.__init__(self)

        layout = QGridLayout()
        self.setLayout(layout)
        self.lst = ["---","Bouleau","Chêne","Sycomore","Houx","Buis"]
        self.combobox = QComboBox()
        self.combobox.addItems(self.lst)
        self.combobox.currentIndexChanged.connect(self.combobox_changed)
        layout.addWidget(self.combobox)

    def combobox_changed(self):
        pos = self.combobox.currentIndex()
        self.combobox.removeItem(pos)
        
app = QApplication(sys.argv)

screen = Window()
screen.show()

sys.exit(app.exec_())