go to account, click edit friends, then list friends, and there's a cross next to names, just click that and it unfriends people. if you block them too, they can no longer even find you so may just assume you've left facebook rather than unfriended them
Go to their profile page. Scroll right down to the bottom and on the left hand side there are links for "Report/Block this person" and "Remove from Friends".
If you just want to defriend someone then click on the "Remove from Friends" link -- you'll be asked to confirm.
If you want to block someone (this stops them being able to find you on a search or send you a new friend request) then click on the "Report/Block this person" link, then check the "Block this person" box and click "Submit."
(You can also block someone directly by clicking on the "Report" link next to a message from them in your Inbox or by going to http://www.facebook.com/settings/?tab=privacy§ion=block, entering their name or email address in the appropriate field and clicking "Block")
Thankyou all! I just want to defriend not block really. Mostly loads of folks i knew from school but weren't really friends with - and they post ALOT! I think i went a bit mad when i first signed up...