"Excommunicating" the Mafia is a PR move for the Pope and the Catholic Church,and of course it is really a symbolic gesture.

To excommunicate someone,you have to identify them. A generic statement like "the Mafia is excommunicated" is meaningless.
What are they going to do,have guys at all of the churches checking for Mafia ID cards?

It's like saying that anyone who reads the Sunday Comics before the Sports Section is excommunicated.Unless you admit it,how would they know?

To refuse to allow a "Mafia member" to enter a Church or to partake in the Sacraments,you have to publicly "out" him as a Mafiosi. Lots of luck on that.