Yes, the HobbyCNC boards operate with just two pins (per axis), Step and Direction. These are TTL level signals.
Yes, the stepper will lock when power is applied - if you've set the motor current potentiometer per the instructions (a quickie calculator is here: hobbycnc.com/sp_faq/missing-steps-motor-not-behaving-properly/) - scroll to near the bottom of this FAQ for the calculator.
in fact, go ahead and read that whole FAQ top-to-bottom for ideas to check.
24V will be plenty to lock the motor such that you couldn't turn it by hand. Depending on the size of the motor, however, 24V may not be sufficient to get the desired torque/speed.
Contact me thru the HobbyCNC website and we'll get you going.
BrianV, HobbyCNC