Early on, while everybody is still fairly low on the tech tree (no spearmen for Romans, no heavy cavalry except for bodyguards) I found out that the best counter to light cav is cavalry of your own.

I have no problems letting enemy cavalry charge my deep-ranked hastati as long as a unit of equites is flank-charging the enemy cavalry.

In an open mobile battle I also try get two of my equites unit onto a single enemy cav unit -- usually it routs immediately the second charge hits it...

Fey