How to remove old Ipod from Itunes?

Hi.
Have bought a new Ipod Classic 80g as wanted more space.

Used to have an old ipod mini 4g.

How do I remove the association of the mini from itunes?

Now when I buy music from istore it still thinks it is for the mini not my new classic so I have to enter my apple id again to authorise songs for the classic. It then says I have used up two of the 5 devices I'm allowed. (1 for Classic and 1 for Mini)

So, how do I tell iTunes I no longer want to sync with the old mini?

thanks.

Ed