If you mean the swipe gestures, then go to your Settings app and, on the General tab, turn Multitasking gestures to OFF.
that's the only thing I know of to turn off. Multitasking is not something you can turn off - it's integral to the iOS. You can, however, shut the apps down (that show in the multi-task bar) by selecting them individually or by using a Cydia tweak such as WeeKill or Remove Background Kill.
Marilyn