How can I make traces end in squares in EAGLE CAD?
As far as I'm aware, there is no way of doing so.
However, a workaround that I commonly use, is to stop the wide trace just short of the pad, and connect the final 0.2mm or so with a thinner trace to prevent the curve going beyond the size of the pad.
An example, showing the traces as partially filled so that you can see each line segment:

Note that the fatter trace overlaps on to the pad, so on the finished board, it will be as if the thin trace is not there.
For more complex connections, you could also use a small polygon to make the final connection.