I wouldn't recommend it considering users are not stored in the Catalog table.
Here is a link to a good series of blog posts explaining the Reportserver schema.
http://sornanara.blogspot.co.uk/2011/05/ssrs-reportserver-database-tables.html
Make sure you know exactly what you are doing before you start modifying and deleting tables in the report server DB.
And then I 'd still recommend you don't do it.