How to automatically remove an item in jcombobox when I already select it?

This is the submit button code:

JComboBox cb1 = new JComboBox();
	Object[] row = new Object [4];
	JButton btnSubmit = new JButton("SUBMIT");
	btnSubmit.setFont(new Font("Tahoma", Font.BOLD | Font.ITALIC, 11));
	btnSubmit.setBounds(35, 153, 84, 23);
	panel_1.add(btnSubmit);
	btnSubmit.addActionListener(new ActionListener() {
		
		public void actionPerformed(ActionEvent e) {
			row[0] = txtfieldname.getText();
			row[1] = txtfieldemail.getText();
			row[2] = txtfieldphone.getText();
			row[3] = cb1.getSelectedItem();
			model.addRow(row);
		}
			
		
	});

This is the table code:

table_5 = new JTable();
	scrollPane.setViewportView(table_5);
	model = new DefaultTableModel();
	Object[] column = {"Name","Employeed ID", "Phone No.", "Schedule"};
	model.setColumnIdentifiers(column);
	table_5.setModel(model);

This is the data of jcombobox:

cb1.setBounds(114, 113, 94, 22);
	panel_1.add(cb1);
	cb1.addItem("6:00-8:00 AM");
	cb1.addItem("8:00-10:00 AM");
	cb1.addItem("10:00-11:00 AM");

My concern is if i select the first option in the jcheckbox I want to removed it completely so It will not choose again.

After you clarified that you actually meant JComboBox instead of JCheckBox this clearly makes more sense. I quickly put together a minimal reproducible example, which contains a JComboBox including your timeframes and a submit button. On the click of the submit button the currently selected timeframe will be removed from the combobox.

public static void main(String args[]) {
    SwingUtilities.invokeLater(() -> {
        buildGui();
    });
}

private static void buildGui() {
    JFrame frame = new JFrame();
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    JLabel label = new JLabel("Select your timeframe: ");
    frame.add(label, BorderLayout.WEST);
    JComboBox<String> comboBox = new JComboBox<String>();
    comboBox.addItem("6:00-8:00 AM");
    comboBox.addItem("8:00-10:00 AM");
    comboBox.addItem("10:00-11:00 AM");
    frame.add(comboBox);
    JButton submitButton = new JButton("Submit");
    submitButton.addActionListener(new ActionListener() {
        @Override
        public void actionPerformed(ActionEvent e) {
            // add row to your model
            // then remove the selected timestamp from your box
            comboBox.removeItemAt(comboBox.getSelectedIndex());
        }
    });
    frame.add(submitButton, BorderLayout.SOUTH);
    frame.pack();
    frame.setVisible(true);
}