There are many threads on this topic, but I suggest that you do a quick search on:

throwaway ticketing
hidden city ticketing
back-to-back ticketing

(which will help you understand both the meaning and the penalty involved)

The summary:

throwaway ticketing is *fine* on UA
hidden city ticketing is *not fine* on UA, but rarely caught
back-to-back ticketing is *not fine* on UA, and more often noticed

<G>