Is there any way to limit trades to only the amounts specified in the "Trade Amount" box?

I've noticed that if I only want to trade with 10Btc (say), if I have a total of 30 in my wallet, and the Drop-loss level is hit, it's possible that all 30 would be sold in 10 + 10 + 10 increments (presumably assuming there are 3 ticks all below my drop loss?  Is that how it works?)

Is the mechanism to create another wallet and put all the coins I don't want traded into there?