Rule 10.1.1 "The Phasing Player is under no obligation to conduct the voluntary attack after he has placed the Target marker - he may later choose to cancel the attack"
This sentence it NOT supposed to be in the rules: remove it from the booklet entirely. Thus, you CANNOT cancel an attack. This line was added by error by GMT and was never part of my original rules.
There is a slip of paper that was added in all the games to that effect.
If it's not included in your game, there is a one-page errata sheet posted in the "downloads" section here on BGG.
Sorry about the mishap.