Follow the following easy steps and you can block friends in Facebook chat:
1) Login to you Facebook Account. Click on the Chat option in the right-bottom of the Facebook page, a box will appear with the list of your online friends. This displays all the online friends of yours.
2)Now click on the Friend List in the top of the box. Here you can create a new list by giving it a name and then press ENTER. You can create as many list you like.
3)The chat box will appear with new friend list(s). Drag the online friends on the list. You can select which friend to keep in which group. Drag the friends which you want to block in the same group. Friends will be seen below the their respective list name
4)You will see a oval icon with white and green colour beside the friend list name. When pointing the pointer it shows Go Offline option, click there and you will be offline for the friends in that list. Literally, those friends are blocked for chatting. Again click the same icon, they will be unblocked.