Also a welcome to this wonderful place from me :)

To stop the previews playing, look in the Girls Store tab. Any cards not bought yet should be there. I have my 'Sort' settings to show the cards by date of release in descending order. This will show the last released cards first at the top.

Now any previews you have downloaded should have a blue 'Preview' flag on the thumbnail. Hover your mouse cursor over this and you'll see a an 'X'. Click this and the preview will stop playing until you manually activate the card again :)

Download speeds obviously depend on your connection. My cards usually take about an hour each when it's working correctly and in my case, usually there are only TWO active downloads at any given time.