How brave are you? I have had to drill a hole
through teh grub screw sideways at an angle
through the wire hole in the posts before. then I was able to use a dental pick in the new hole to slowly back the screw out. its a pain, It could ruin your deck, but it works.
Or, pull the positive post out, drill into the grub screw a bit and then you can use a vise to slowly press a slightly larger hex bit into the hole. I say pull the pos post so that your not putting excess pressure on the post and insulator while working on it.
Possibly contact Steamcrave and see if you can send it in to them... they could probably help you out.