I say Diss. When I first saw the link to the Disboards, I thought it was going to be about "dissing" Disney and I was going to get all upset about someone dissing my Disney! LOL!
From all the examples I could find, in the English language, whenever you have a word with an ISN in it, it is always pronounced as a Z sound:
Isn't
Buisness
Disney
If it is a word with a ISB in it, it is pronounced as an S sound:
Disbranch
Disbud
Disbelieve
Disbar
Disboscation
Disbind
and so therefore i would also include disboard.