I'm not sure, I have it on my 14 MM, if my game would actually work I'd try it on Goldbrickers for you.
I have not seen it manifest a -fly so far (am at lvl 33 now), but I do know that the immob power does 
can pretty confidently say that it doesnt have a -fly aspec, i use it tons of times on my MM and never have i shot anybody out of the air with it.
"god, how many devs did hami have to sleep with to get ED?"
Total Cat @Stagefright
Force Bolt doesn't have -Fly either, but if you hit some flying PvE mobs with it they will descend gracefully.
It's a strange one, but I'd expect the answer is no. My Ninja/Poison MM is languishing so can't do any testing for you! (too busy with MZ and ARAD).
Doesn't it have exactly the reverse effect as Power Boost? Power Boost increases Fly speed and Jump hight and speed, so weaken should reduce fly speed and jump hight without completly canceling them (in theory).
I really should do something about this signature.
Just remembered to check while I was online with my MM, according to the ingame text and the power description it just reduces their Accuracy,Damage, and secondary effects (heals,stuns,holds de-buffs, that kinda thing).
Hope that helps.
The only poison power I can think of that has a -fly type thing would be Paralytic poison, the single target hold power (which rocks), I use that quite often to bring down flyers.
I have made a lvl 40 robot/poison MM on test server event, and Ill try to test. I dont think any of the poison powers have -fly รก la air superiority. Im not sure that weaken even has a slow component.
So far Ive used neurotoxic breath to slow down the opponents.
ps. Poison rocks! 
Its just very,very squishy..
Does it do -jump -fly?
Im using CoX builder and it says so in the description on there but im not sure if it is accurate because it doesn't say it in the ingame power description.