Not sure exactly if that's what you want, but there are a few things you can do, you can select the vertices you want flatten by using the scale tool and pulling the axis you need down flattening them, the tool itself has World, Object and Normal directions to help the Tool flatten the vertices ,edges and faces to the direction you want.
You can also place another plane next to it and snap the vertices by pressing "V" to the vertices of the other object.
There might be other ways but its all I have from the top of my head.
