If any of your sims get to socially deprived, the Tragic Clown will come. To prevent this in the first place, talk a lot. If you are extremely stupid and don't have anyone to talk to, type in the move objects cheat (everyone knows that by now because of the million other submissions), Go to build mode, click on your sim/sims and dedelete him/her/them. If you did not do any of this, or did but was too slow and that god damn clown comes, delete it.