constructor(af: AngularFire) {
  const ref = firebase.database().ref().child('item');
  ref.onDisconnect().remove();
  const myObjObs = af.object.list(ref);
}

const onDisconnectRef: FirebaseOnDisconnect = new Firebase(url).child(path).onDisconnect();
onDisconnectRef.remove();

onDisconnectRef.cancel();

typings install --save --ambient firebase

import { AngularFireDatabase, FirebaseObjectObservable } from 'angularfire2/database';
...

  user: FirebaseObjectObservable<any>;

  constructor(db: AngularFireDatabase) {
    this.user = db.object('/users/test');
    this.user.set({state: "online"});
    this.user.$ref.onDisconnect().update({state:"offline"});
  }

constructor(private db: AngularFireDatabase) {
      let url='/items/item';
      const ref = this.db.database.refFromUrl(url);
      ref.onDisconnect().remove();
      const myObjObs = db.object.list(url);
    }