Anyone can but the bar would be raised for the casual troll. I am not so sure about the method of requiring pre-approval but once someone gets $n level of down votes we have a fair indication they are up to no good.
Perhaps life can be made harder for trolls and spammers with little chance of hurting true seakers of knowledge. The community certainly seams to get their number pretty quickly and flag them with heavy downvoting. At least a goal worthy of discussion
Pereant, qui ante nos nostra dixerunt!