Yes, there is a seperate button for blocking, actually :D.

Try the left Ctrl button and see if that helps :D.

edit: keep in mind that the Barbarian can block as well but due to the fact that he doesn't use shields, his block relies more on your reflexes and the ability to time the block just as your opponent is about to strike you.

The other classes should have the ability to block with a shield raised until they are fatigued. I have no idea as to how the Amazon blocks as I have yet to play as her :D.

Yup, as Judas said you can block with Ctrl. It breaks opponents balance for a while and give you time make a combo or few free hits. IMO blocking is to tricky (timing/reflex dependant) and I always preferred dodge-hit-dogde-hit approach.