SQL logs filling up with "Setting database option Compatibility_Level" messages
Only a couple of things come to mind.


Hack sp_dbcmptlevel and add a "IF suser_sname() = 'Your SSRS Account' RETURN" to the very top of it.
Add a DDL trigger to the database that simply rolls back any alter database statements.


There's no way to intercept the database call and prevent it from being sent to the database engine.