I have a dialog like this:
GtkWidget *dialog = gtk_dialog_new_with_buttons("Spell Checking", NULL, 0,
GTK Widgets usually have a property
Sensitive, when it's set to false, the widget grayed out, i.e. an user couldn't interact with it. There's a function for that purpose
void gtk_widget_set_sensitive (GtkWidget *widget, gboolean sensitive);
Also you could try to set it directly, like
myButton->Sensitive = false.