There is no such problem. The United States conducted special studies on this topic, which showed that Bitcoin is used for illegal purposes only in a few percent of cases. First of all, this is explained by the fact that it is only relatively anonymous. Criminals in their activities use coins with complete anonymity, such as Monero, Dash and others. And in general, this is not a serious problem for cryptocurrency. Almost everything can be used for illegal purposes. This should only be a problem for law enforcement.