Geeetech

I have a Prusa I3 Acrylic.

Both X and Y belts are rubbing the sides of the idler bearing brackets.

This is causing them to leak a black liquid. I can't get the belts to stay in the center on the bearings, they always move to one side or the other. There is a lot of play in the brackets, I am able to move them from side to side by hand, changing the angle at which the belt meets the bracket.

Is there a way to keep this from happening?

Slight rubbing is not a big deal.
The belt is not in the center , that is also not a big deal, if they do not influcence the moving of axes.
The only improvement is adjusing the assemly of belts.

I installed this design and filed the edge but that didn't help, here is the belt guide, change the z axis according to the gap you want to close, be aware that these will introduce slightly more friction if you didn't adjust the z-axis of this thing properly, or you could just install 1 side of them only

http://www.thingiverse.com/thing:788085

Slight rubbing is not a big deal.

Yes it is, over time the belt will saw thru the metal bearing holder.