The Tax Office has a list of everyone who makes a professional payment to me, as they apply withholding tax which they then deposit in my name.
So if you owe the Tax Office money, they can just say 'right - we'll get it direct from source'. They issue a payment order to your payers, demanding that they pay the State instead, as it has legal priority. 'Distraint' is the specific term, I believe.
But as I say, they apply a scattergun approach, in case Client A maybe owes me nothing that month, or just 50 euros. And there is no immediate procedure so that once Client A has settled the debt directly, Clients B, C, D automatically have that distraint order repealed. They ended up sitting on a few thousand euros of my cash until in their sweet time they decided that in fact they ought to hand it back.
It's pretty brutal.