How to get adult child out of the house

Technically, if an child is not paying rent, they are a guest, not a tenant. Under the law, a guest may only remain so long as the property owner allows them to. Further that permission may be withdrawn at any time, for any reason. If it is withdrawn and the guest does not leave, they become a trespasser and the police should help remove them. So in theory, there is no need for an eviction action.
 
However, the police sometimes refuse get involved in these situations, feeling that if someone has been living there for a period of time or claims some right to live there, that the courts should determine whether they need to leave. If that event the parents would need to bring an action for "ejectment" (which is basically an eviction for non-tenants) in order to remove them.
 
It should be noted that if the child is supposed to pay rent, which can include paying utilities or otherhousehold bills, then even if they have not in fact paid these amounts, they may still be considered to be a tenant. In other words, the obligation to pay, even if not honored, creates a tenancy. In this case, they would need to be evicted and could be on the grounds of nonpayment of rent. Alternately, if they are a tenant without a written lease, then they are a month-to-month tenant, so the parents may give a 30 days notice terminating the adult child's tenancy, and then evict them if they do not go.