You can find suppression lists under your organisational profile.
You can reach your organisational profile by navigating through organisations on the sub menu and through all relevant sub organisations until you come to your organisation.
Create List
You may wish to create a list of people who would like to be suppressed from all SMS Notifications. To do this, you will need to create a suppression list.
Manage Lists
Once you have created a list you will be able to add users to the suppression list. Remember these suppression lists may stop users from receiving important information (especially workflow instances) so please make sure you have selected the correct user.
If you need assistance with customisation check out customisation.