Is the issue about “locking” the brake in a stealthy manner for theft “obstruction”? Or is it about locking the brake for stability? If the latter, the traditional leather toe strap, cinched around the brake lever and handlebar is superb! Or one can strap it around the downtube and front rim for an even more stable set up. If it is a stealthy attempt to foil a possible grab and ride, one can jack the front derailleur into the chain to make pedaling away nearly impossible. I wouldn’t rely on anything short of a U lock , cable, or Euro style rim lock on the rear wheel as a way to foil a grab and run/ride, though.