Can a figure with reach choose to not use reach in an attack?

The situation is for Imperial Hospitality, where there is a prisoner that can only be attacked if there are no heroes next to him who can be targeted by the attack. So with a royal guard in the following formation:

XPH p=prisoner, H=hero, G= guard , X=empty space

GXX

If the guard has reach then he'll have to attack the Hero. Otherwise he won't have to.

I would say no.

From the rules:

Since you legally can attack the hero, you must.

Of course, if you had 1 more movement point, you could move up one space and then attack the prisoner.

The intention is that in this case, reach is not optional. Per the mission rules, the guard could attack the hero, so by suffering the strain, the hero can make it so.

Reach is not optional, it is always active. Just like Mak's Covert is always active (until he becomes wounded).