If you have Auxila, they are, according to Ace Cataphract, extremely good at stopping them if you throw three or four at one. Unfortunately, Auxila are useless againt anything else.
Alternatively you could have skirmishers with you, put them in loose formation, and have them throw javelins.
If you're lucky enough to come across Mercenary Elephants, get them. They cost about 6000 denarii but they're worth it. They just trample straight over chariots, killing them with one hit.
EDIT: Also, you could have your legionaires set to Fire at Will, which will take down a *lot* of them if they throw fast enough.
EDIT2: Oh, and one more thing: DO NOT PURSUE THEM WITH ANY KIND OF CAVALRY OTHER THAN ELEPHANTS. When they come up to the chariots, the scythes cut straight through the horses' knees.
sig[This message has been edited by Bk 101 (edited 09-27-2005 @ 09:37 PM).]