I respect your approach to this after your son admitted his part in damaging the table. A two week ban from the club isn't too strong a punishment really.
As for the lack of punishment for the other items damaged, if no-one has come forward then it's hard to appoint blame on hearsay and 3rd party versions of what went on. I'm not saying that the club are right in this, but they can't prove who did it... or can they ?
I think you are over reacting a little by saying you'll not be back though. You and your son have done what was necessary. The club either delve deeper into this or they have to let it lye as it is.
