I don't see how they do. Their method of catching these drivers are is not fit for purpose. There are a few in my town, one is operating for over 2 years. if they are clever enough they will never be caught. Even if they are caught they get a tiny fine and some keep doing it after conviction.
I can think of better ways to catch them and stop them.
Get the guards to keep pulling them over and taking names of the passengers, build up this evidence and then use it against them, no one is driving people around all day for free.
Revenue could watch them and give them a big bill for unpaid tax, keep giving them these bills.
it is hard to watch some fucker driving around your old customers and undercutting you because they are paying no tax and pay maybe 400 euro for insurance.