The horns regrow at a rate of about 12cm a year, so would have to be cut back regularly - also the poachers are happy to cut out the part that is below where it can be cut, below the surface - like having fingernail roots dug out.
If the poachers could get paid work that meant that poaching was not necessary to them, and they had a pride in their native animals that went deeper than a desire for the money they could make from them, it would go a long way toward stopping poaching.