i have never used this myself, but according to an old GT wiki the tesla staff can drain the power from a quantum suit. however, it didn't work in the past and i don't know if it was updated since. you could try it.
...Damnit. People really need to get the word out somehow that the Tesla Staff always is/was/has been a joke item, Greg advertised it as a really expensive thing that can one hit quantum, but it is/always was "completely untested", meaning that it was never intended to work in the first place, and in fact you can not even hit entities with it.
I play on a war server, and really this thread has a good question that is hard to answer. If you're using voltz (with IC2 added in of course, or just using ICBM in general), sonic missiles are among the most effective things I know to kill quantum, although that is somewhat abusing a glitch; strictly speaking from a Direwolf/Mindcrack standpoint though, one of the things I have found to be most effective is an EV electrical trap with MFSUs sending up-converted EUs into a loop to more MFSUs, as previously stated, MFFS works well, an IC2 Nuke (or 4) can get the job done easily, but with a massive amount of collateral damage if not properly contained, and the last thing I know of that has not been previously stated is... sand or gravel, for some reason the IC2 Hazmat suit protects against sand or gravel suffocation after they are dropped on top of you, but the quantum suit does not.