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
This is the basis of most "security" measures- not ensuring absolute security, but decreasing the likelihood of an undesirable event occuring by, as Random_Walk pointed out, "raising the bar" in such a manner as to create the least possible interference with the affairs of regular users.
"One is enough. If you are acquainted with the principle, what do you care for the myriad instances and applications?" - Henry David Thoreau, Walden