MacOS – Symantec Auto-Protect

You want to remove "write permission" from the file, so it cannot be written to. If you block MobileMe from writing to it, you will also be blocking the application from writing to it, which could cause problems.

Via the command line:

chmod u-w file.plist # u-w means "Remove Write from User"
chmod u+w file.plist # Undo: add user write permissions back again

From the Finder, do CMD+I (Get Info), and under "Sharing & Permissions" at the bottom, set it to read only.

If you need to write to the file (perhaps for the application to work properly), you can temporarily make it write-able, or approach this with a different solution. For example, you could use DropBox, link the file to DropBox from all your Snow Leopard computers, and just not link the file on the Lion computer.

The Xprotect "system" is not really an AV engine as such. It looks for a very limited number of text strings, or "signatures," in specific types of files. For this reason, it is far less useful than a real AV program (up to you if you think one is necessary). In addition, it only checks files downloaded with certain apps: Safari, Mail, Messages, and third-party apps that have activated a setting to use Xprotect to check their files as well. Also, it only sets a flag on files, and depends on user interaction for protection. So an un-savvy user may see the dialog warning that a file is dangerous, but still click the necessary button to open it.

As you say, it doesn't protect against BitTorrent downloads (though specific BitTorrent clients could use the feature), and doesn't protect from files copied from any kind of removable media or files copied over a network.