Any baby store will sell devices for securing furniture to the wall to prevent it from tipping over. They also sell similar hardware for those in earthquake country.

Make sure you screw into the structure of the armoire, not the [usually] thin back.

you can use L brackets to attach to the top of the cabinet, then to the studs.

It's easiest to do this with screw eyes and a bit of tie wire. One eye in the cabinet one into a stud; orient it toward the hinge side, and one set may be all you'd need. Bt a set at each corner won't hurt.
The brackets are problematic for getting access to drive the screws, if it's close to the ceiling. Screw eyes will just require slightly careful measuring.
Casey