I don't think its as clear cut as that, you have to take each one as it comes.....different people react differently.....again its being firm, but fair...shouting might work with some people, but with others it will just get their backs up. Just be confident, self assured and dont take no for an answer. I'm not a big guy myself but Ive never had any trouble I couldnt kerb.....
At a punter day Im assuming you mean, best way in my experience is to make it clear that any team caught cheating, even just one player will default the game or lose points to the other team....works like a charm trust me...its treating them like school kids, but the thought of letting their entire team down and upsetting everyone else usually works....
If you have to ask them to remove their hopper, put their marker on the floor, or even take it off them if they are a real idiot....if its not against your sites safety policy you could send them back to the safe area. Or of course just tell them if they are seen firing you will deduct points from their team.....
Another option I just thought of but have never used is putting opposing players back in play for each person found cheating/firing into play....