step 1) You Knock Prone someone, through either a spell or a physical maneuver (same thing).
effect of step 1) the NPC now has the Sticky Aspect "Knocked Prone", meaning a free tag for the person who applied the aspect (or whomever he hands it off to) and it stays around until countered by a second maneuver.
step 2) the NPC goes to act, before the PC can act again. The NPC can do anything he could do on the ground... fire a gun, cast a spell, ect, ect..., or to do a maneuver to counter the sticky aspect of knocked prone. As nothing is keeping him prone right now (the spell had no duration, ect), this is pretty much an athletics check of 0, meaning he's going to stand up if he wants. Note, however, that this technically costs him his action (as he is doing a maneuver). Alternatively, as this is really an uncontested, pretty much unrolled action, it sounds perfect for a supplemental action, costing him a -1 to whatever his action is this round.
but the GM and the player both have the potential to spend a FP to compel the NPC to fail any action here. The PC may choose to do so when it becomes apparent that he is going to lose his free tag, but he will also try to calculate how likely the GM is to compel the aspect (and thus save a fate point).
So as a GM, you have to decide to compel or not. Generally, I base this on the narrative I want to write. Is this an easily beaten bad guy, that I know the character will quickly defeat? Compel it, because it's a cooler story if the baddie just flops around and gets pwned. Is this a character that I think the PC is going to intimidate for information, and leave alive? Compel it, slipping the bad guy a FP for a later scene. Is this guy supposed to be tough, or fighting to the death, and present a real physical challenge to the PC? No compel, because it doesn't fit the story.
So, if it doesn't get compelled, or is compelled and bought out of, lets say the NPC stands up and the aspect is gone. Otherwise, it gets compelled, the NPC loses an action, gains a fate point and we move to..
step 3) the PC goes again, and tags the aspect for a +2 to punch the guy in the nose. woot.
step 4) identical to step 2, because the aspect is sticky, and thus hasn't ended yet. The brilliant player says, hrm, I will compel the same aspect again, on and on, until I am out of fate points, because by then I will have pummeled him to death. So he compels it. The NPC spends the fate point he just got and ignores the compel, then stands up.
Stun lock = impossible, because the first compel gives the NPC a way out of the second compel.