I have looked at this on SSRS for SQL Server 2008, hopefully it will not be too much different for SSRS 2005.
As other people have said, you can add another group or user to SSRS with admin rights, then delete the BUILTIN/ADMINISTRATORS login. However, SSRS is written so that anyone in the local Administrators role can grant themselves rights if they want.
This means that if the BUILTIN/Administrators login is not present, someone with local Administror rights cannot access anything in SSRS by virture of their local Administrator rights, but they can re-instate the BUILTIN/Administrator login or add their own login with whatever rights they wish. After they have set up their rights, they can do what they want.
This means you cannot block local administrators from doing things in SSRS, but you can have a site standard to say they should not access SSRS. If you get management backing for this and someone with local Administrator rights forces their way into SSRS then it becomes a company disiplinary task to deal with it.
The SSRS 2008 Configuration Manager GUI woks slightly differently. The GUI can only be run by someone with local Administrator rights, regardless of the rights local Administrators have in SSRS.
IMHO SSRS security is a bit broken but still workable.
Original author: https://github.com/SQL-FineBuild/Common/wiki/ 1-click install and best practice configuration of SQL Server 2019, 2017 2016, 2014, 2012, 2008 R2, 2008 and 2005.
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