I delete songs from my iphone 5 but when I sync, they come back. How do I keep them from re-syncing to my phone?

I'm out of space on my iPhone 5, so I have to be choosy of what songs I have loaded. With that, I can't add new songs because when I delete existing music, they come back when I sync with iTunes. I don't want to delete them from iTunes, but I don't want them to so sync. I do want to sync music so when I purchase new songs, they also get loaded into iTunes. Help!