The decision comes after the court had already increased the sentences of the four defendants who were found guilty earlier in November.
The four members of the ring, considered only footsoldiers, were convicted by the Ljubljana District Court in November 2012 the on the basis of evidence collected in Italy.
But the court acquitted the remaining 13 after the panel of judges threw out wiretapping evidence from Slovenia and Serbia that it said had been obtained unlawfully.
The ruling came as a shock considering that wiretapping evidence was seen as highly conclusive, highlighting the popular perception about the inefficiency and unjustness of the justice system.
The charges in the Balkan Warrior trial were the result of a sting in May 2010 in cooperation with Serbian police and the US Drug Enforcement Administration.
They came in the aftermath of an arrest in Uruguay in October 2009 of two men trying to smuggle 2.17 tonnes of cocaine to the Mediterranean.