Gaeilgeoir @Vimiostie11 To block a Sim card, take a look here: Block SIM, activate eSIM/SIM card | Swisscom To change the user of a mobile sim card, the old and the new user have to go to a Swisscom Shop with an identitication document (ID, passport)
LeylaG Hello @Vimiostie11 and welcome on the Swisscom Community 🙂 The contract holder can initiate the takeover from this link. Hope it helps!