As I said before, it's so hard to pin these bastards. The only answer is to increase sentences for those that are caught, to create risk.
In general I advocate much higher sentences for crimes that are difficult to police.
So for these scums I advocate 5 years first offence, 20 years subsequent. The US is good at long sentencing. There needs to be a lower grade facility to make prison life more unpleasant. Bring back shackles.