On Editing in the RIchtextBox i want one combobox to be disabled

Please see my comment. It's not good to answer for all variants. Always specify exact type. There is more than one type called RichTextBox. Always use a full name.

Also, "on its edit" might mean different things. It can be the event Focus or TextChanged. Think about which one you need to indicate "on its edit" and disable the ComboBox instance in its handler:

myRichTextBox.TextChanged /* or .Focus */ += (sender, eventArgs) => {
    myComboBox.Enabled = false;
    //or
    //myComboBox.IsEnabled = false; //always specify UI library!
}

See:
http://msdn.microsoft.com/en-us/library/system.windows.forms.richtextbox.aspx[^],
http://msdn.microsoft.com/en-us/library/system.windows.controls.richtextbox.aspx[^].

—SA