Fresnels tend to spill a lot of light. Always using barn-doors helps a
lot - close the doors so they're just touching the edge of the beam.

For some models of lantern an accessory called a "snoot" is available.
This looks like a top hat with a hole in the top, and sits in the colour
frame runners.

For profiles or PC lanterns many spill problems can be fixed with black
Cinefoil ( http://www.rosco.com/film_video/cinefoil.html )

Be careful about blocking too many ventilation slots on a lantern -
they're there for a reason.