Something like this happened to me recently with an online buy of DIAO. I also didn't know these rules existed and i also was pissed off.
If the spread is too wide according to some algorithm, they do not let you trade. Not sure what it is but at a guess might be 25% to 30%. In my case I think it was 0.7 to 1.1c. I simply placed a limit order up near the top of the spread then traded at market.
