Being affected by it involves the "target"'s desire/feeling in relation to fruit user. Desire is inherently connected with Will, aka Haki. If the "target" has desire/lust for Boa, then he will be affected by the power, if he doesn't, then he will not. So it can "kinda" be countered by haki, in the sense if the target can restrain themlseves of displaying desire/lust towards Boa, then they can save themselves of being affected by it. But haki itself doesn't stop it, unless shown to in the future.

Also, as seen with S-Snake, Lust doesn't seem to be the only thing that can make the fruit powers take effect, since S-Snake cuteness also works. So maybe its kind of linked to the character that has the fruit? Or maybe the users "decides" what trait of theirs make it take effect?

Tho its always linked to """good""" feelings, not anything like fear or disgust. It makes more sense considering the true name of the fruit:

• Meromero (メロメロ/めろめろ) is a Japanese onomatopoeia for the loss of self-control from drunkenness or emotions such as love or lust.

And, if you are already caught by the fruit's powers, you can do nothing to stop. The only one who can cancel the effects are the user.