If you are doing this, then you are using a *VERY* old PAC, with *VERY* outdated wording.

Optional/Non-optional hasn't been a thing for a couple years now. I highly recommend you get the latest version of the rule book and PAC (most recently in the Guardians of the Galaxy starter or available for download from HeroClix.com).

He was completely, 100% incorrect.

Powers that are activated by a specific action assignment (like Incapacitate) are only activated if you specifically say you are giving the character that action to activate that power. You literally can't "accidentally" use Incapacitate (or Psychic Blast or Pulse Wave or Quake and so on).

While I will certainly never tell anyone that it is a bad thing to be specific with your action assignments, in this case anyone who tells you otherwise, that tries to tell you that you did use Incap even if you didn't want to, is either misinformed or trying to cheat you.

From the Rulebook, page 18:

ACTIVATING POWERS AND ABILITIES
Powers are in effect when they appear on the character’s combat dial through the stat slot. Abilities are always in effect. Powers and abilities activate in a number of ways:
• Some game effects require that a character be given an action. These effects only activate when the character has been given the action specifically to activate the game effect. The game effect then overrides the normal activity of that move, close combat or range combat action.
• Some game effects activate as a result of something specific happening. The game effect will use the words “when” or “if” to describe the scenario required to activate the power or ability.
• Other game effects are always active or otherwise specify when they can be activated.

Well, you can't move on with the attack until they have decided whether or not to roll. Taking no answer as a decision to not roll is not a good thing to do, because some players will argue they were still contemplating their choice or assumed nothing needed to be said to imply a yes. If you move on to the attack without knowing with any amount of certainty the decision your opponent made and you were aware they had a decision to make, then you cheated.