A cassette (7) for a printing ribbon (8) comprises a casing (11) accommodating a supply spool (23) which has a tube (26) on which the printing ribbon 8 is wound. An anti-unrolling element (9) of plastics material comprises two portions (31) and (32) of elongate shape, which are connected by means of an integral elastic hinge (33). The first portion (31) performs a guiding and positioning function and is accommodated by means of a slot (34) within the casing (11). The second portion (32) has a blocking function with regard to the tube (26), and is movable on the outside of the casing (11), by means of the elastic hinge (33) from an inoperative position in which it is aligned with the first portion (31), to an operative position in which it is rotated through 180 o and is substantially in superposed relationship with the first portion (31). The second portion (32) is provided with a hook element (48) which, in its operative position, engages the tube (26) through an opening (53) in the casing (11) for positively blocking the supply spool both during storage and during transportation of the cassette (7), to inhibit unrolling in a simple and reliable manner.