I know it's easy to say that gambler needs to be responsible and know how to control their spending but I agree with you that it's hard to do if we were in their position, But it's really up to them, Gamblers are the only one who can help their selves even if there's a lot of advices and reminders that they've received, if Gamblers didn't know how to control and manage their selves, I bet they will ended up having a gambling addiction, Worst, it can ruin their entire life.
First things first is the willingness of the person to help himself, if that is already present the chance is high of curing itself, though if addiction is already deep then professional help and guidance from someone who you really trust will help in boosting your self-will to quit away, there are many factors that can really affect your decision making when trying to un-tie yourself into gambling, we do all understand addiction and it's not that easy to remove if you have it inside you.
It takes time and solid determination to help yourself, but it's always doable if you really believe that your changing pace will help you to solve your own mistake.