As the card is written, it is a bit of a mix of optional and mandatory, I believe. Air Power quoted the relevant section of the rulebook, which states that the player can choose to spot 0 if they so choose unless the card says some variant of "can spot", which this doesn't.
However, there are two costs to the action: "At the start of the maneuver phase, spot another
Man and exert this minion to add a threat." The attempt to add a threat is going to happen with or without the player's input; it's no more avoidable than the text of
Dwarven Heart. First the player spots (or doesn't spot) a
Man, then
VWM is exerted, and then if both of those actions were performed, a threat is added. Imagine if
Dwarven Heart stated "at the start of each of your turns, exert bearer to heal Gandalf." This would very clearly mean that you are intended to exert and this exertion happens whether or not you want it to, but if it does happen, it pays the cost of a secondary action, that of healing Gandalf. This is precisely the same as this card, except that there are two separate action costs, and one of them happens to always be optional.
Thus, while the player may or may not choose to spot another
Man, you can't actually avoid the exertion that is automatically going to occur. So the best you can do is prevent the threat from being added, if for some reason that's what you want.