Many of the asteroids may not be viable targets due to the fact that they spin too fast to allow for spacecraft to get close or land on them. However, with this new technology accessing these asteroids might also be possible in the future.
Students of the University of California (Santa Barbara) have used laser beams to slow down a small spinning piece of rock. By timing the laser burst the rock could be slowed down and even reversal could be achieved. The method has only been proven on small scale basalt samples, so would require significant up-scaling.
It is well known that many asteroid (or comments) change course due to radiation or heat from the sun. There is thus no reason this would not work on a larger scale or on Asteroid or Comet for that matter.
The laser must be a lot more powerful though…
…alternatively one could use large mirrors or reflectors to generate the pulsed energy needed, thereby harnessing the power of the sun.
More details on the method, called Directed Energy System for Targeting of Asteroids and exploRation (DE-STAR) can be found here.