how to remove alien in the game ?

@kirstey08 

You can't get rid of aliens completely when you have the Get to Work pack but you can minimize their presence.  You will get some aliens initially and from doing certain things as you play.  Playing the Scientist Career will generate the biggest number of them.  Alien night at the bars will generate some.  Going to Sixam will generate some.  Other than those on Sixam, most will be in disguise so you usually don't see them around in their alien form and may not know they're an alien unless you become friends or develop a romance.  And you still might get alien abductions.

The only thing you can do to get rid of the ones you do find, is deleting them in Manage Households.  The game will generate the aliens it needs for certain things again so it will be an ongoing thing to keep them out of your game even if you avoid doing the things that causes the game to generate them.

There are a few mods that will help with controlling the number of aliens in your game if you play on a PC or Mac.  MCCC does have some options dealing with aliens.  I've never used them as I like having aliens in my game so I'm not sure what all of them do.

Hope this helps.