Ref to http://devnet.jetbrains.net/message/5276609#5276609,
- open File | Project Structure | Modules | <your module> | Dependencies tab
- click Add, select New Library (of any level: module, project of global)
- give any name to your library, click 'Attach classes' and select requred SWCs, click 'Attach sources' and select 'sym_src'
Now files (.as) in sym_src are uneditable at all. I'm using IntelliJ 10.5.
(in 10.5, they are 'Add SWC Files' and 'Add SWC Sources').