A pipe cutter will work but they tend to leave a "ridge" on both sides of the cut if you tighten them too much. You may have to file it off the outer circumference of the shortened seatpost to get it to fit the frame properly.

BTW, measure how far up from the bottom the current "minimum insertion" line is and be sure you can insert the seat post that far after you shorten it. Typically the distance is around 75 mm although long MTB seatposts may recommend more due to the expected increase in leverage when a lot of post is exposed.