Выйти из процедуры по клику на кнопку

 Private Sub ComboBox_SelectedIndexChanged(ByVal sender As ComboBox, ByVal e As System.EventArgs)
'....
    End Sub

Private Sub Button3_Click(sender As Object, e As EventArgs) Handles Button3.Click
'....
End Sub

   Private Sub ComboBox_SelectedIndexChanged(ByVal sender As ComboBox, ByVal e As System.EventArgs)
        Dim Ind As Integer = CInt(sender.Name.Replace("ComboBox", "")) - 1
        For I As Integer = Ind * 5 To Ind * 5 + 4
            Controls("TextBox" & I + 1).Text = (Format((Data(sender.SelectedIndex).T(I - (Ind * 5)) * Var4184), "0.##"))
        Next
    End Sub

Private Sub Button3_Click(sender As Object, e As EventArgs) Handles Button3.Click
        ComboBox1.SelectedIndex = -1
        ComboBox2.SelectedIndex = -1
        ComboBox3.SelectedIndex = -1
        ComboBox4.SelectedIndex = -1
        ComboBox5.SelectedIndex = -1
        ComboBox6.SelectedIndex = -1
        ComboBox7.SelectedIndex = -1
        ComboBox8.SelectedIndex = -1
 
    End Sub

       For i As Integer = 1 To 8
            Me.Controls("ComboBox" & i).SelectedIndex = -1
        Next

        For i As Integer = 1 To 8
            DirectCast(Me.Controls("ComboBox" & i), ComboBox).SelectedIndex = -1
        Next