Resizing a video in Premiere without scaling
Create a sequence that is 720x720, drag the clip on to the timeline, and in the canvas window use the arrow tool v to double click the layer which brings up the layer controls. Position it the way you want, export.  I'm pretty sure Premiere doesn't scale non-uniformly by default.  

If it does for some reason, you'll probably want to use the conform options to adjust how it is conformed to the sequence.

You'll need to use a custom sequence setting since none of the presets will match your 720 by 720.  You'll also want to make sure you use square pixels (1.0 pixel pitch) if you are looking for a perfectly square output.