Swing twice before they can swing back after a block.
And it is very, very possible with proper footwork; weapon hardly matters for it though. Opponents using Unbalanced weapons are easier to spam, since they take longer to recover from blocks.
"oh, my opponent has an unbalanced weapon! Express ticket to spam town!" 1 second later, you get a kill when they think they can swing back at you. If you do it right. And No, I am not telling you how because I despise it.
If you play NA you can watch Castor, grey [formerly knows as guts], Cyranule or a few others do a guaranteed double-swing spam
footwork technique. If you don't pay full attention to the subtle footwork while blocking, you'll lose the duel.