How can I tell jackson to ignore a property for which I don't have control over the source code?

You can use Jackson Mixins. For example:

class YourClass {
  public int ignoreThis() { return 0; }    
}

With this Mixin

abstract class MixIn {
  @JsonIgnore abstract int ignoreThis(); // we don't need it!  
}

With this:

objectMapper.getSerializationConfig().addMixInAnnotations(YourClass.class, MixIn.class);

Edit:

Thanks to the comments, with Jackson 2.5+, the API has changed and should be called with objectMapper.addMixIn(Class<?> target, Class<?> mixinSource)

One other possibility is, if you want to ignore all unknown properties, you can configure the mapper as follows:

mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);

Using Java Class

new ObjectMapper().configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false)

Using Annotation

@JsonIgnoreProperties(ignoreUnknown=true)