You really shouldn't use the physic engine, make your own everything, might be it gravity or collision, it's really easy, you have lot of tutorial for it.
Anyway the solution of your problem is very simple ; unset the physic or whatev on the object you want to make rise from the bottom of the screen. When's done, you just have to code a very simple movement code :
Step Event :
y-=spd
spd being NOT the built in variable, but the number of pixel that the object will move each step. Hope it will help you