Of course there is a major change from previous sanctions in that the only major navy concerned with imposing oil sanctions on Iran is the USN: The RN is active in the Gulf but only to protect British flagged ships. The isolation of the US on this issue is clearly shown by the refusal of the Gibraltar authorities to respect an US request/injunction (the legal force of this is not declared) to hand over an Iranian tanker previously held - but only for breaking UN, not US, sanctions. Clearly the refusal of the European, Indian, Chinese and Russian authorities to take part is making it a lot easier for the Iranians

I think the effort might be more effective if some people were tracked, too. Not every skipper is willing to take part in smuggling operation. I suppose there is a list of "usual suspects" here. They might be using cell phones and credit cards and travel with their own legit passports.