Tools -> Options -> General -> Drawing Window
There is a check box there called enable autoconnect. Uncheck it.
With regard to the one right angle allowed, just change the connector style right clicking on the connector, and changing the style to curved.
Edit: If you are actually talking about lines, and would just like a visio 2007 way to do what the answer did in visio 2010, go to Tool -> Snap and Glue -> General -> Currently Active
uncheck snap.