Adium does no specific processing of data when copying. The only way* to not copy the data is to not select it. If you want to copy several lines at once this means that you wither need to use the transcript viewer with timestamps hidden or use a message style that doesn't display the timestamp. on each message. Of the bundled styles, yMous has an option do this.
Since the message styles are nothing more than HTML, CSS, and Javascript, it should be possible to create a message style that will do what you want. IT would be easy to simply apply "-webkit-user-select: None" to the class "x-time". Unfortunately, a bug in Webkit prevents this from working properly (https://bugs.webkit.org/show_bug.cgi?id=80159).
*(A second option would be to use Javascript to process the copy buffer however you'd like. To the best of my knowledge nobody has ever done this, but it's theoretically possible.)