A taser can be any number of things. The simplest is a maneuver; put the aspect "tazed" on them, and then invoke-for-effect to trigger a compel - a fairly reasonable compel being "You can't act this exchange." Of course, this feeds them a fate point, but them's the breaks.
The other option (and the one a PC in my game went with) is for it to just be a straight up attack spell - damage, weapon rating, and all. It's just that the consequences from it are things like "Tazed" (usually a minor) up through "cardiac arrhythmia" (an extreme consequence), and with take-out results that are generally non-lethal (assuming no prior heart conditions in the target).
A block probably isn't the best way to go, simply because it can be overcome by any skill, depending on what the person being blocked decides to do. (For example, you set up a tazer-based block with a value of, say, 4, and it'll break if the guy tries to intimidate you and gets a 5 on intimidate. I mean, it's workable, but the mechanics feel kinda silly to me. Though, for something that's actually a block on all actions, see the grapple rules, or the example spell "Orbius" on YS294-295.)