Abel: Why do they always hold up crosses in vampire movies?
Yvonne: In vampire legends, it is believed that crosses ward off vampires.
Abel: What does that mean?
Yvonne: Well, they can't touch it, so it prevents them from hurting you.
Meaning: The phrasal verb "ward off" means to try to keep someone or something away. A "No Trespassing" would be used to "ward off" trespassers (people who shouldn't be in a place), just like crosses are used in the above example to "ward off" vampires. "Ward off" can be separable, but usually only with a pronoun, as in the following example:
Abel: Huh. I don't know much about vampire legends. Is anything else supposed to ward them off?
Yvonne: Yeah, lots of stuff. Garlic, holy water, sunlight, fire. They're scared of a lot of stuff.
Abel: I guess that's good to know if I ever run into a vampire.
Meaning: You can see an example of a separated "ward off" in Abel's question, when he asks if anything else can "ward them off."