For me, a 15% tax on gambling can never stop most gamblers from gambling, it will take above 50% to make enough gamblers reconsider or limit their gambling activities. This 15% is on their citizens but it is worse with
gambling operators in the country, they have to part ways with 55-58% of their income for various taxes in the country. That's huge.
But this recent ban on online casinos is not meant to gain more taxes but to make sure that those online casinos gaining from their citizens who are not registered in the country also pay taxes to the government.
Tax will never stop gamblers from gambling even if the government imposing so high such as 50% or more. They will still playing gambling and try to win the money. But if gamblers can thinks realistic, they will reconsider to limit their their gambling activities or even they will stop their gambling activities because they think the percentage is not make sense. 15% for their citizens can be done but the government needs to educate their citizens not to playing gambling too often so they don't have to pay tax or lose their money.
When the governments get more taxes from the online casinos, that will be additional income for the country. The government can ask to that online casinos that if they still want to operate in their country, those online casinos needs to follow the regulation and pay the taxes.