On a trainer, like the Avistar, and at the airspeeds they fly at, the misalignment of the stabilizer will likely go unnoticed. As long as it's flat horizontally, you shouldn't have any problems flying the model.

To correct the problem, the best way to remove it is to use a thin-bladed saw and cut just above and below the stabilizer, cutting the glue and not the stab. Some sandpaper on a stick to clean up the slot, and then some balsa shimming to take up the space the saw left.

It's a pain, but can be done if you're careful.