How to disable the option of edit when a checkbox is checked?

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C#

I have a windows application that runs sql scripts. The scenario is that I have given user the option to use existing database that is there in system. Like:

Use existing Database: (checkbox)

If the user clicks on it, he will get a dropdownlist of database existing in the system.But, the problem is he can still edit in the Dropdownlist\textBox. Actually it is a combobox.
I wanted to disable the part where he/she can edit when the checkbox is checked.

For reference :

private void chkUseExistingDb_CheckStateChanged(object sender, EventArgs e)
       {
           CheckBox cbUseExistingDatabase = (sender as CheckBox);
           bool IsChecked = cbUseExistingDatabase.Checked;
           if (IsChecked)
           {
               if (ValidateMandatoryFields())
               {
                   txtDatabaseName.Text = string.Empty;
                   txtDatabaseName.SendToBack();
                   cbDatabaseName.BringToFront();
                   cbDatabaseName.DataSource = GetDatabaseNames(txtUserName.Text, txtPassword.Text, txtServerName.Text);
               }
               else
               {
                   cbUseExistingDatabase.CheckState = CheckState.Unchecked;
               }

           }