You can easily manage the premium services of all lines belonging to your MyGoMo account at any moment on https://www.go-mo.ch/mygomo
In the Account details section, click on Blocking of services and tick the premium voice services and/or premium SMS services you wish to deactivate.
Please note: You will then no longer have access to any premium services and will no longer be able to use your mobile phone to pay for services (e.g. Selecta machine, parking tickets etc.).