The only way to seal the bore so that the DO's prison is the same as before the drilling is to use balefire in such a way and with such strength as to nullify the creation of the original hole. Think what could be achieved for instance if lanfear (Mieren) were to come back to Randland and then be caught by such a powerful bolt of balefire. Lanfear was instrumental in the creation of the hole in the DO's prison and if she was eradicated by balefire then the hole in the DO's prison would never have existed. This scenario fits in with the name of the series too as a loop in time or at least a major paradox would have been created.