plus500 is not dealing with bitcoins. They just allow to trade and speculate about the mtgox price. So, it does not impact the price, since it does create any order on the market.
But what happens if the market price is falling and most of its customers are shorting bitcoins?
The broker will obviously lose a lot of money ...
Probably ban some high profitable accounts, invoking their TOS. Also close the possibility of trading during high volatibility market.
I don't know exactly which mechanisms they are using, if they hedge the positions or not, I think that it is somehow balanced between bull/bear, and they can support some downsprings.