You probably need to set

resource_limit = TRUE

in your parameter file. This activates profile resource limits.
May be missing something here, but doesn't the fact that people are being disconnected after a period of idle time suggest that this parameter is already set. All the databases I've seen have resource_limit set to FALSE and nobody ever gets disconnected.

Are you sure that you don't have multiple profiles setup and you didn't check the right one for the users who are disconnected after 30 minutes?

[editing] sorry Dapi, your post wasn't there when I started typing!