For starters, if you're talking about street opponents, (not street bosses) they just disappear for a few seconds and then reappear in a different place. If that didn't happen, then there would be a shortage of monsters to defeat, and that would definitely cause problems (especially if you had to collect a lot of things.)
If you're talking about bosses, well the quests ones (if defeated) are just like us really. You defeat them, they talk to you (or thank you for freeing them or something) and then they reappear back in the tower (or occasionally the street) and you can defeat them numerous times over and over. If they didn't have that then the term "farming" wouldn't exist.