Ban OTC? How are you going to do that?

Same way you do with gambling, price fixing cartells or any other contract the government wants to get rid off. Ban the action and give it suitable punishments, make it possible for government to seize assets if the action is revealed, and refuse to let the criminals use the governments resources to uphold the contracts.

Illegal activities will always still happen, but it will be riskier and less profitable. And thus probably less common.

Sweden's finest (and perhaps only) collaborative, leftist e-newspaper Synapze.se