find the savefile, which is in you user directory, which is usually in C:\Documents and Settings\<user>\My Documents\OpenXcom\data or C:\Users\<user>\Documents\OpenXcom\data , open it with a text editor, text-search for "genUnitType: STR_FLOATER" and find the one that has "health" higher than 0, change "health" to 0 or, even better, "stunlevel" to something higher than "health".
start OX, load the savegame, hit "end turn"
EDIT: okay, ninja'd by yankes. you get the idea, though