Unless you have a matt ballon which I never ever have seen, you will have a reflection.
The point is what reflection you can handle.
If you do not want a hotspot, use as Romero Ninov said a softbox. If you do not want a rectangle as a catchlight (which can be pretty nice) use a light tent.
Make a box of PVC pipes and construct the walls of vegetal paper (waxed paper is too transparent so do not use them) Make a round hole to sneak the camera lens into the tent.
Now uniformly lit the walls of the box. Probably two intense lights at 45° angles horizontally and vertically on both sides a little far away to make the light uniform.
What you will have is a biiiiiiiiiig catchlight, soo big that you will not see it is a catchlight.
One variation is making a conical tent, like a teepee, so the light is even more uniform.