make angle flat again

[videogames2theend]

so I made a square awhile ago in my scene and copy and pasted it a lot, I just realized that two sides of it (front and back) are not flat, I estimate their at 1 or 2 degree angle. I was wondering how could i get them to be totally flat again?  

[lukasvogl]

if your planes are still separate object with transforms you could simply select all and scale them down to zero in their up axis in the channel box