As far as I have tested now, and as "FXExecution" only works with "switchmove", there is no way you can prevent the moron from jumping on his feet again in some after-death spasm.
Here is a suggestion for a possible work-around :
_mymoron switchmove "FXExecution"
~3
_mymoron setdammage 1
_mymoron switchmove "FXExecutionDead"
At that stage, it's your choice :
* either you point the camera elsewhere (hoping it's a cutscene...), and you'll have seen the guy shot (the prob' is the same with "FXExecutionDead", the guy jumps back on his feet) ;
* or you combine these moves with other ones, the only one that I've found being "TreatedLyingToLying", but the move isn't that fluid (the advantage only being that he lies dead for good).
I realize that I've assumed you wanted the guy whacked. Hope it's the case...