OK, I misunderstood what you were saying. Let's be sure. Disconnect the motor completely and make sure that the three phase wires can't touch each other. If you still can't turn the motor, there's a short circuit between two of the phases. The place to look is where the cable comes out of the motor. If it does turn, you have to find the short at or further up from the connectors. It's very simple to find where the problem is with a meter. Do you have a meter? You shouldn't be considering an electric bike conversion without one.
When a 3 phase motor is shorted in some way (or in fact almost any electrical motor, DC or AC, single phase or 3 phase is shorted), it resists turning to some degree, but it will turn.
This used in many sittuations to brake and prevent runaway, it is called "Dynamic Braking!" There is a reasonably accurate description on Wikipedia for anyone interested.
But it happens only if you try and turn it quickly, that the resistance to turning will be noticed more. The motor has to produce some voltage as a generator, which it can only do while turning, and the short produces a huge "load", braking the motor.
If power was still being supplied, in some obtuse manner, then it could possibly feel locked. Though I personally have never experienced that, only read about it.
But this sounds more as though the motor is "mechanically locked" (If I understood the OP correctly), almost as if something is obstructing the wheel/motor.
I am totally guessing, but maybe possibly a motor bearing is badly damaged....causing the problem. Or some other failure in assembly!
I would be MOST interested to find out what the final cause was eventually, such problems interest me greatly! This one is to my mind, quite unusual....
I tend to feel that a replacement unit may turn out to be the easiest and best fix, as I myself would not want to ride on a bike that had exhibited such a problem as described, without knowing the "whole story!"......
Best of luck for a speedy fix,
Andy