IS, press R, or choose the crop tool in Develop, in the local adjustments panel at the top. Click the vertical double headed arrow at the far right of the 'Aspect' area. Select 1:1 from the drop down list. Ensure the padlock is 'closed'. (Edit: I agree, the display spacing there leaves a little to be desired, it's hard to see tha that double headed arrow goes with aspect ratio.)
(If 1:1 is not available, [I think it should be by default], you can add it, with 'Enter custom ...')
Oops, simul-posting with Don again.