How to block streaming in fort proxy 400E for specific Users in the domain

Which user method authentication are you using? FSSO, explicit proxy authentication, ... ?

Check this tech tip for explicit auth:

https://community.fortinet.com/t5/FortiProxy/Technical-Tip-LDAP-authentication-on-FortiProxy/ta-p/193069

And for FSSO:

https://docs.fortinet.com/document/fortiproxy/7.6.0/fortiproxy-authentication-guide/9081/using-single-sign-on-with-the-fsso-agent

If you are looking for the easy and simple solution then avoid FSSO.