The best way to do it is to go on the actual server and log in to the "Reporting Services" in SQL Server Management Studio. Under there, go to home and then find your report. Right click on the report you are interested in seeing the security for and click the "Permissions" page. I don't think it is possible to delete/add with an SQL query, but I could be wrong.
Good luck!
~Brett
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Web/.net Programmer & DBA
Central PA