Constraint is important and true, but isn’t an exact-opposite type of match for “force multiplier”.
A “Charley Fox”?
I can’t think of a hypothetical situation where “degrader” would not be a good enough word to mean this. Degrader gets my vote, if a single all-purpose good-enough word is what’s needed. It works as-is, or can be used in “force degrader” to mirror the positive term.
Handicap or handicapping agent maybe?
The senator asks what it costs. The general asks what it does. The soldier asks what it weighs.
Heh, I can’t see that name without fond memories of the worst general in history thread. That was an amazing thread! 
I’ve heard it more Charley Foxtrot, though Tango Uniform works as well.
That’s just what I was thinking of, too! Here it is: Worst Military Leader (game thread) - The Game Room - Straight Dope Message Board
I actually read the whole thing - again!
What a fun game that was. Some great high-level discussion. 
Indeed it was. The Dope at its best.
If caused by another combatant, “debuff”. If caused by the powers that be, “nerf”. 
I don’t think “handicap” works very well, because of its ambiguity. In a person, handicap is often used as a synonym for disability, but in a broader sense (especially games), handicap means a method of bringing competitors’ chances of winning closer to equal.
Impotencer.
Weakenifier.