Go into trade chat, and on the bit where it says attack/peace/surrender/alliance/treaty you will also have declare war. Just select and send that, then blow them to bits. If it isnt there and you have an alliance with that civ just select break alliance then declare war.
It's not there in DL.
You'll just have to attack someone/something, Denny. You could also try leaving transports/warships in someone's space for several turns, to try to get them to attack you.
