In that case then creating a portable threshold effect would be the best way to achieve the same effect within the DFRPG rules.
And how would that work, exactly?
Anyway, here's the revision. It's not really any different from what you wrote, it's just reformatted and clarified.
POWER NULLIFICATION [-varies]
Description: You have the ability to deny your enemies access to their magical powers.
Musts: You must decide what the base Refresh cost of this Power is before any upgrades.
Skills Affected: Discipline
Effects:Power Nullification. As an action, you may attempt to nullify the Powers on any character in your zone. Make an opposed Discipline roll; if you win, your target must choose Powers of theirs with a total Refresh cost equal to the base Refresh cost of this Power. Those Powers stop working for the rest of the scene.
Diminishing Returns. If you use this Power multiple times against a single character in the same scene, they get a +1 bonus to their second defence roll and a +2 bonus to their third and so on.
At Range [-1]. You may use this Power against any character within your line of sight.
Area Effect [-2]. You may target an entire zone with this Power, draining the Powers of everyone in the zone except yourself. If you have the At Range upgrade you can target any zone in your line of sight, if not you must target your own zone.
Three more questions.
1. How does this interact with rebate Powers like The Catch? If you have Supernatural Toughness with a Catch of cold iron, does ditching Supernatural Toughness count as losing 4 Refresh or 1?
2. How does this work on someone who can't lose the specified amount of Powers? If you have Mythic Strength and no other Powers, what happens when I drain 3 Refresh of Powers from you?
3. What happens on a tied roll?