The ability to turn off casting calls is a sensible feature with an annoying side effect.
Casting calls are very often extremely detailed - describing the nature of a shoot, rate, location etc. When you reply to a casting call, all those details aren't usually reiterated in your email conversation - because they're already stated in the casting call itself, and the conversation surrounding the job has a handy link supplied which lets you click through to the original call.
Problem is when the photographer or model turns off the casting call, once it's filled. It makes sense to turn the call off - to make it disappear from the casting call list, as any further respondants would be wasting their time.
At this point though, all those shoot details become lost to those involved in the shoot itself. As a photographer signing up to a group shoot for instance, I would no longer be able to see the studio name and shoot details, I would just get a blank page with 'Sorry, but we couldn't find the casting call you were looking for, it may have been turned off by the creator'.
Solution: Turning off casting calls should remove the casting call from the casting call list, but the link provided in any emails discussing that job should remain active.
Include the text of the casting call as a quotation at the end of the first response any interested party sends to the casting call creator. This would have the benefit of 'locking in' the casting call details, even if they were changed later.
I think this is a very important oversight - not just from a convenience point of view... the casting call itself forms part of the 'contract' regarding the shoot, and it should be available for reference at a later date.