Since the bitcoin users are somewhat hard to track then the government can tax the platform the users are using base in their country. Maybe they can add an additional tax every time a user buy or sell bitcoin or any other cryptocurrency so that in this way all transactions are accounted for. Though there may be instances that the user can still get away with being taxed when they are using platform not registered to the country they are residing. Our government for instance do not have regulations yet to cryptocurrency therefore no applicable tax because they cannot still classify it so the user can still freely spend the coins and earn without being tax but it would be different later because the government noticed the growing crypto enthusiasts in our country so they are making moves now to regulate it.