Yea, I can defend my planet, but if he keeps doing this I'm only defending the rest of hte game, which is rather boring. I wanna attack! ;p
Doesn't take much to defend, 250 steel a turn. Save the rest for a fleet, once he's gone, build it and then send it out to attack! Even better, use his own ships against him! Once you can build a fleet 2x his size, build it, set it to pirate, watch his ships become yours!