A high tax could prevent people from getting addicted by giving them fewer chances to gamble if it is their real intention but the current gambling addicts are the ones who will be affected especially if they don't really care about the tax they pay for each of the session they attend. It would be a real burden to them and the long-term effect might be good or bad knowing that they can easily go bankrupt in gambling and adding high taxes will make a toll on them.
Tax shouldn't act as a way of persuading someone not to do something, even if that is gambling. Taxes should only be introduced where necessary, and while I would assume a lot of people would argue they are included in a lot of places where they are not necessary, they are generally for contributing to the community. For example, in the UK we have the national health service (NHS) which tax money goes towards, as well as road maintenance, and all that good stuff. Taxing gambling wouldn't make much sense, when we already get taxed enough on our income. We have a tax free allowance up to around 12k ish, and then from then on out we pay 20%, 40%, and 45% dependent on how much we earn. I know in the USA tax works difference, and in general you guys get taxed a lot less, mainly due to not having a public health service, at least a tax paid one.
In fact, you don't usually get taxed on losses at all, since it wouldn't make much sense. So, how would losing money on gambling be eligible for tax in the first place? They would only be taxing successful gamblers, not the opposite way around.