Instructions
Build a room in your house for the kidnapped Sim in Build mode. The room can be as large or small as you like, but should be big enough to fit the Sim inside. The room should be accessible from only one door.
Bait the room with something that will attract the Sim's attention. You cannot control guest Sims to make them enter the room, however having something entertaining like a TV or video game system will make the Sim enter on his own accord.
Invite a Sim that you want to kidnap over to your Sim's house by clicking on a telephone and selecting "Invite Sim Over."
Greet the Sim when they ring your doorbell and invite them into your house. Wait for the Sim to make his way into the bait room.
Enter Build Mode when the Sim goes into the bait room. Click on the door and delete it. Alternatively, you can build a bait room around the Sim when he enters your home by constructing the walls around him.
Return to Live Mode and the guest Sim will be unable to leave your home.