Well I guess one way to kill them is to NOT get charged by them. If your unit has a good number of attacks then you should be able to kill them, they have a crappy armour save! I'll go through the armies and pick out units that can kill them.
Space Marines: Well generally alot of things can kill them. Far range weapons are probobly the worst. Stuff like flamers, series of boltguns. Somebody said Landspeeders, Assaults charging in can kill most. Furious charge would be good to charge in, seeing that your guys would attack first.
IG: heh, all you need a a crapload of lasguns and your good to go. Again, flamers and other assault/ rapid fire should do the job.
Nids: guants can do decent damage with their cheap guns, and huge numbers. Again long range ( carnies, Hive Tryants) won't do the job, especially with Viel of tears. Charge in with genestealers( and possibly with a broodlord) adn wipe them out.
Tau: again, rapid fire range from fire warriors should do the trick. Not sure what else for Tau.
Dark Eldar: Again I'm not too familiar with these guys. Pssibly just close-mass firing from whatever can kill them.
Eldar: Dire Avengers, Banshee's on the charge( your own Harlies on the charge
) should do the trick.
Necrons: Pretty much just warriors mass firing. Maybe Pariahs can charge and make them hit on 6's if the rolling is bad for Harlies?
Pretty much anything else falls into those catagorys. In sumary there are 3 rules you can follow to avoid getting killed by these space clown
1. Close range/ mass firing is a good idea. You don't want to fire too far, especially if your opponent has Veil of tears with them.
2. NEVER NEVER NEVER let them charge on you. With that many attacks on the poor suckers, good luck surviving.
3. If all else fails, CHARGE! Better you then them