Do you have access to your database - like through phpMyAdmin? If so, you may want to go to your pmtext table and order the PMs by date. It looks like all the PMs you are talking about will be the earliest dated (1969? Did we even have forums back then?!?!

) See if the PMs are all from the same user id. If so, you can do as Medina suggested and remove all the PMs sent by the one user through your Admin CP.