Catalyzes the decarboxylation of carboxynorspermidine and carboxyspermidine. Carboxynorspermidine is decarboxylated 20-fold more efficiently than carboxyspermidine. Exhibits some activity with L-ornithine, but shows no activity with L-arginine, L-lysine or meso-diaminopimelate. carboxynorspermidine + H(+) = CO2 + norspermidine carboxyspermidine + H(+) = CO2 + spermidine Homodimer. Belongs to the Orn/Lys/Arg decarboxylase class-II family. NspC subfamily.