Yes i actually also would like to know that. It does'nt seem to be possible to deactivate thru my Arturia account, but i think it should be possible to do so, when a computer is broken.
It look like it's only possible to deactivate thru ASC from the computer where the ASC is installed. It could also be good if one thru ASC could manage licenses for other computers including deactivation.
Is it possible to get deactvation for a computer by contacting Arturia support? That could be tried, but it would be easier if it was possible to deactivate yourself, even for a dead computer.