How to mass-delete text messages by partially matching phone number on iPhone?
I haven't heard of an App that can delete text messages at all, but I can see it's usefulness in a situation like this.

Furthermore, I don't think there exists a solution to group senders with (partially) unknown numbers, if you new the numbers an ugly and time consuming solution would be to add all possible numbers to one contact, then all messages should hopefully be kept in the same conversation. Your only solution right now seems to be to manually delete the messages :(