I have about 25 machines and a collection of served databases. Whenever a client opens an FM file locally, there's a message displayed that says the file cannot be shared because sharing has been turned off.

That's all fine and dandy, but how do we turn off the message? It's getting old...

You are receiving this message because to share databases FileMaker Sharing must be turned on. That means if someone else happens to open the same database it will warn them about the fact that they won't be able to share the file. There is really no way of getting around this unfortunately. If you turn FileMaker sharing off you will no longer be able to have multiple people access the file so you probably don't want to do that. Sorry I couldn't be more helpful.