My question is after you beat the game can u still go around and kill the various monster and stuff ? or is the game over and you have to restart it? cause i hate the fact that i beat BG2+Expan and at the end you dont get to keep on playing.

Also are some areas of the game world capable of respawning monsters?

When you desfeat the final boss or complete the final task the game will end like almost all games do..

Yeah, the majority of all games ends after the final boss. So does NWN, but there are two expansions and literally hundreds of modules out there to download. And online gameservers.