Regulating crypto currencies is actually extremely simple: you ask every "official" user of a crypto, to keep an official bookkeeping for inspection. You may even make a block chain application for that ! In that bookkeeping ledger, you are required to declare all crypto you receive, and from where it comes, for what purpose. That's all. The crypto can be as anonymous as you want, and as decentralized as you want, the simple thing is that you're not *supposed* to own any crypto of which you've not declared its possession.
Mind you that nobody is going to come and lay their hands on your computers to verify whether you don't possess secret keys of one or other crypto. No. But the point is that you cannot spend *publicly* any crypto that you didn't declare its origin from. You can hold as many bitcoin, DASH, Monero, ZCash, Shadowcash or whatever. But you cannot spend a single one of those in an official place if you didn't declare where it came from, including exchanges. Of course, you can use foreign exchanges to swap your bitcoin for monero, then transact them to yet another exchange and so on. But in the end, if you want to spend them, say, at Indian Airlines to buy an airline ticket, and you pay say 1.5 BTC for the ticket, Indian Airlines will report that YOU, Joe Sixpack, bought an airline ticket for 1.5 BTC. Now, officials can go and check how many bitcoin you declared. Maybe you declared that 3 years ago, you bought 5 BTC on an exchange. OK. Leaves you 3.5 officially useful BTC.
If ever you try to pay 3 other things at Amazon India, and the total amount passes 4 BTC, then officials will (automatically) know that you were fraudulent: you've paid in total more than the 3.5 BTC you declared, with BTC. ==> you're in trouble.
Imagine that you had obtained in fact extra BTC because you speculated on an exchange. No problem. You simply had to report it ; in fact, the exchange will do it for you when you withdraw coins: "Joe Sixpack has received 5 BTC from our place".
This doesn't stop using crypto in an underground economy. But you cannot get this crypto out of the underground circuit to use it officially. Exactly like "dirty cash".