Display the UserName in the Report Parameter who ever accessing the report

I have created a report,It has 5 parameters Startdate,Enddate,TransactionType,EntityType and User. There are 2 EntityTypes(AA & CC). when User selects EntityType parameter, User Parameter will be enable. I have set default value  for User Parameter with below expression :

=User!UserID  (It returns the ID of the user running the report)

But this value should not be editable(means user can modify the value in the User Parameter after preview the report.) in report preview.It should be readonly. Is this possible ?

another query is If the EntityType is AA  then I want to pass "NA" in the User Parameter otherwise(If the EntityType is CC) want to pass UserName as value in the User Parameter. I tried with the following expression but no luck.

=IIF(Parameters!EntityType.Value = "AA","NA",User!UserID)

Can anyone help on above queries:

Thanks ,

Visu