Coins can ONLY be sent from whatever address they were sent to... When you send coins to an address, they are associated with that address... and that address alone... it doesn't matter that you have multiple addresses in your wallet... they are all technically separate.
So, to ensure you are ONLY sending from a specific address, you need to use the "coins" tab... if you can't see it from the menu select: "Wallet -> Coins"
You will be able to see exactly which coins are in which address. Select the inputs from the address you need to send from, right click and select "Spend"... only those coins should be used in your outgoing transaction (NOTE: this may mean you cannot send the total balance of your wallet, if other coins are in other addresses).
Also, you may want to disable "change addresses", so that any change will return to the original address. Otherwise, the change will go to a different address in your wallet. (Tools -> Preferences -> Transactions -> Make sure that "use change addresses" is UNCHECKED):