How to delete M2M relationship?

In user model class I have the following:

public class User implements Serializable {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    @ManyToMany(mappedBy = "attendees", cascade = CascadeType.ALL)
    @Cascade(org.hibernate.annotations.CascadeType.ALL)
    private Set timeslots = new HashSet();

And i would like to delete the timeslots..i've tried something but doesnt work, as follows:

public static boolean deleteUserTimeslot(EntityManager em, Timeslot ts) {

    EntityTransaction transaction = em.getTransaction();
    try {
        ArrayList<User> attendeeList = (ArrayList<User>) ts.getAttendees();

        List<User> attendeesToRemove = (List<User>) getAllUsers(em);
        transaction.begin();

        for(User u: attendeesToRemove){
            for(int i=0;i<attendeeList.size();i++){
                if(attendeeList.get(i).getId()==u.getId()){

                    em.remove(u.getTimeslots());
                    break;
                }
            }       
        }
        transaction.commit();

        return true;
    } catch (PersistenceException ex) {
        //Rolling back data transactions
        if (transaction != null && transaction.isActive()) {
            transaction.rollback();
        }
        logger.error("Error making database call for update timeslot status");
        ex.printStackTrace();
    } catch (Exception e) {
        logger.error(e.getMessage(), e);
        e.printStackTrace();
    }
    return false;
}

Every entity object has to be removed by a separate invocation of the remove method. Removing a set of time slots in one invocation (as in your code) is not supported by JPA.

If Timeslot is the owner of the many-to-many relationship then the other side will be updated automatically when User instances are loaded again from the database (you can use clear or refresh to enforce update).