I think it was shared here once how to unlock...
Tried it 8 months ago or so..but again give it a try...here's the method...
1-go to Google images search
2- type facebook+"name of 1st friend" copy & paste from blocked account
3-search , you will see the picture of the 1st friend in the 1st 3 results
4-if you didn't identify the picture, repeat the same
5-if you didn't identify the friend.simply skip the picture and you will get another one (always you have 2 skips/picture)
6-after you identify the 5 pictures facebook will ask you to change your password and email password
7-change your facebook account password and confirm that you change your email password (even you don't)
8-now every thing is OK
9-if you see this message(no verification method is available now) just logout and wait 1 hour to login,if you failed to verify the photos, facebook will give you 1 chance to verify 5 pictures every 1 hour.
Credits: To BHW (Can't remember the exact thread)