Sizere is right, the directional antenna will only work if it is implemented by the ISP... and they won't do that or they will cease to be an ISP and simply become a bridge for him.

Sizere: Help us out here....

got some very interesting traffic
.... and the items of interest are????? Is the MAC address different? How about the TTL's. Post us an example of the attackers traffic and traffic that is entirely your own and lets see if we can prove to the ISP that this is a different machine altogether.