Home Sharing, but without syncing purchases?
Home Sharing is done under a different Apple ID than the one the computer is authorized for. I have my computer registered under my ID (with automatic downloads on!), and Home Sharing set up under a family ID.

To do this correctly:


Make sure you are signed in to your ID under the Store menu.
Log in to the shared ID with the appropriate command under the Advanced menu.
Voilà!


Alternatively, you don't even need to use Home Sharing. You can play tracks without enabling it, as long as sharing is enabled under iTunes > Preferences > Sharing > Share my library on my local network. Enabling Home Sharing will let you copy files to the local library, whereas this option will not.