simeon1770, you have a few options to go about fixing this. If the arm is light enough, you can simply set the braking pin on the Victor to brake. As I understand it (read: I'm just an alumnus, not an engineer), this produces an effect similar to crossing the leads on a motor. IT makes it very difficult to turn, hence, holding it in place.

If you arm is a little heavier, then a working software-oriented fix may be mounting sensors on the robot (a shaft encoder may be a good solution here). From there, you would need to read the count and, in the simplest case, set a value you want the arm to go to and then a lower limit at which point the motor lifts the arm back up to the desired value (if you set only one value the motor will be rapidly switching on and off trying to maintain that position). If you need more control over your arm, or your arm is fragile, etc..., then a more complex control method may be in order. A PID, or Proportional, Integral, Derivative control may be a good solution then. This option gives you a little more control over the arm, but may take a little time to tune (Also, advice I've found useful is to use only a P control and then add the I and D terms to eliminate error in the P solution after tuning it.).
For more information, the programming side of controlling your arm has also been discussed here.

If you're looking for a more hardware oriented fix, and have the weight to spend, adding a worm gear to the drive-motor gearing has worked well for my former team in previous competitions. Other hardware oriented solutions include adding a mechanical brake or changing up the gear ratio on the arm.

Depending on your situation, a different solution or mix of solutions may be the best for you. Hope this helps.