I've been through this before. Its a long story, so I won't go into the whole thing. In the end the USBC won't do anything. You can bring them up for dismissal, and have a league vote. (I don't recall, but I think you need a 2/3 majority). In my case, most teams felt that the cheating was obvious, but felt expulsion was too extreme. So despite everyone encouraging me to take this up for vote, most didn't have the backbone to stand up to the cheater. On a good note, he stopped sandbagging after that.
In the ABC rule book (old version), rule 17A covered unfair tactics, defined as intentionally bowling under a person's average to gain an advantage in league or tournament play.
I had to right a formal complaint to the President of the league, and list the charges. I presented a very convincing argument with lots of data, and even a re-rate from a tournament. There was literally no doubt he was cheating. The most important factor is whether the other team captains will act, or not.
Good luck if you take that route, and be prepared for some backlash. Doing what is right, isn't always popular.