Shed floor framing out of square 3/4 inch
Use a ratchet strap or a come-along to pull the corners together on the diagonal that measures longer than the other, then, before removing this, secure a sheet or two of plywood down to the floor joists. 

You can use some big eye bolts through the rim joists to give your ratcheting device something to hook into at either end. 

You shouldn’t need to worry about damaging anything. 3/4” out of square means you’re compressing the long diagonal only 3/8”, and nails allow for some wiggle and play in the framing before it is sheathed with ply or boards.

I don’t recommend beating on the thing you’re building. It’s hard on your body, and doesn’t hold things precisely square for you while you do the other work to sustain the squareness once achieved, like laying plywood on the floor.