if the ball has a thumb hole and it is not used; then that is an illegal throw/roll.  The rules are clear, only griping holes are allowed and all of the holes drilled must be used on every throw.

I would say something to the secretary or president of the league; and if they turn a blind eye, I would elevate this to the association.

I would recommend the same.  The bowler probably thinks it's not that big of a deal. But a rule is a rule to me, and I also paid to have some balance holes plugged, so I'd definitely say something.

Good luck enforcing it.  We have bowlers around here that have been reminded by league officers of the rule, only to have the conversation degrade in to near fisticuffs.  Most serious bowlers know the rule and abide by it. 

The casual bowlers who bowl on a sanctioned social handicap league once a week, tend not to know about it and frankly don't care.  They figure "a ball is a ball, the pro shop guy drilled it for me, so he must have done it right.  He told me the hole had to be there to make the ball legal, and now you are telling me the hole has to be filled to make the ball legal.  That makes no sense!"  Yeah, he drilled it 5 years ago when the rules were different.  Rules change.

The balance/gripping hole rule and the dry towel rule have created so much grief it is ridiculous ("But the bottle says Anytime During Competition right on it!").  You couldn't pay me to be a league officer or volunteer at the association level these days.

In our City Tournament, in Team Event, there was a guy that would put his thumb in the ball and throw in warm ups, then he decided to not put his thumb in the ball when the event started. He was caught and told he has to use a different ball without the thumb hole. He started using his thumb hole again. Then he was caught again 3 frames later not using his thumb hole and was talked to again. He claimed he didn't like the feel of using his thumb. He was told that he and his team will be disqualified if it happens again. He said "you gotta do what you gotta do, but I'm not using my thumb."  He and his team was disqualified from the Team Event. He would have placed second in All Events. He also would have placed somewhere in the other events as well.

kiefenstien - Did he use his thumb in the Doubles/Singles or just wasn't caught? My guess is he just didn't get caught.

I really don't know if the rule makes a difference at that level but the rules should be enforced. If not who decides which rules to ignore.

I was bowling state a few years back and they made an announcement before practice started that you were only to take one shot and leave the approach and you should not reset to get a full rack of pins. A bowler on the other team reset every time he didn't have a full rack and wouldn't stop when called out on it. The officials refused to do anything to the bowler and he he got his way. That didn't set real  well with me.

kiefenstien - Did he use his thumb in the Doubles/Singles or just wasn't caught? My guess is he just didn't get caught.

He was not caught in Doubles and Singles but he was asked to be honest during Doubles and Singles if he used his thumb. He was honest and said he did not put his thumb in the ball. So he was disqualified.

My gut says that since his thumb comes out before starting his approach, it's an illegal delivery. Now, if he were to start his approach with his thumb in the ball and then remove it prior to getting to the foul line, it could possibly be considered legal since I don't think there is a rule that says that your thumb has to be in the ball for a specific amount of time. I guess it comes down to at what point the delivery is said to have taken place. Do you have to have your thumb in the ball until your final step? Again, I would think common sense would say yes, but I could see someone trying to find a loophole there.