More about STUN
In Zoiper WEB (npapi) STUN can be only turned off/configured per account in the account configuration.
enabling STUN through JavaScript.
example:
/* Here we enable the STUN usage */
Account.STUNHost = "stun.zoiper.com";
Account.STUNPort = "3478";
Find this information in the Zoiper Web API documentation
If you do not enable STUN through JavaScript at all, STUN will not be used for NAT traversal for your accounts.
At the moment STUN cannot be switched off globally.