If i understand correctly, you want to disable the soap sender authentication for the requests coming through portal?
There is an option to disable the authentication at the adapter level but not at the channel level.
Having said that, can you confirm how are you consuming the PI services in Portal?
If it is custom development object(I assume it is) then you might want to check the possibility of adding credentials as http header while calling PI web service. You can use any generic user for all your web-service calls.
I personally tried this to call a JSP(which requires logon credentials) from Portal and i developed an abstract portal component to pass these details as part of the http header.
These options are also addressed in the below blog -