SRAM BB30 cranks - how to remove crankarm from spindle?
The left arm has no threads because that side is designed never to be separated from the spindle...
From what I can find, that's an M18 thread, pitch unknown but probably M18x2.0mm or M18x1.5mm.
Remove the right arm, put the spindle down through a sturdy surface with a hole no larger than 31mm so the metal part of the left arm has maximum support. Screw a steel rod of the correct M18 threads into the good threads you still have in the spindle. Hammer the end of the rod. The longer the rod the easier it will be to keep it perpendicular to the surface that's supporting the crank.
You can use a deadblow hammer if you have one, or go find an auto mechanic's workshop since they are very likely to have a hydraulic press they use to press bearings into car spindles.