I haven't shot an indoor league with classes either, I normally see that in outdoor 3-D. I have never shot indoor 3-D though. That doesn't mean they can't do it, or make their own rules for their own league if they want to. I would ask him to show you where it says you can't use it. If they have a rule that says you can't use electronic sights then I guess you are screwed. Or if they they have one that says you can't use a one pin adjustable sight, if you have that sort of mount. They might also consider it a scope, although I wouldn't since it isn't magnified.
If it doesn't specifically state that you can't use that type of sight and you are in a "hunter" class then I would think it would fall on your hunting regs. If electronic sights are illegal to use hunting then I don't see where you should use them in a competition with "hunting" equipment. If they are legal I think they are in the wrong.
As far as getting your money back I doubt there is anything you can do about it.
This is one of the reasons I don't like any kind of leagues or competitions. I am not a very competitive person, I just like to have fun doing something I enjoy. And competition just seems to bring out the A hole in people. Some take it way to serious and suck all the fun out of it. Maybe you could ask them if you could keep shooting but just not be eligible to win or something. I have done that before, I do it with 3-D quite a bit because I like using a range finder. I just want to shoot, I could care less about being better than everyone else or winning a stupid trophy. Some people just can't accept that type of mentality though. I don't keep score when I golf either, then again I suck at golf
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I do see their point though if they do indeed have a rule against it. Even if you were not dominating or even if you sucked it still sets a precedence for someone else. If they let you do it they will have to let everyone do it. If it were me and I didn't have rule specifically saying you couldn't use that sight I would let you finish the league then change the rules for the next year.
Good luck,
Paul