You can block your buddy in two ways,
1. Groups list, buddy section, swipe right of the buddy you wish to block. Tap Settings icon to enter settings page and click Block.
2. From buddy settings page. Tap Block button to block the buddy.
Note: The blocked buddy will not be notified after you initiated the block. But once you block the buddy, you will not be able to receive messages from that buddy until you unblock the buddy. The blocked buddy will be removed from your buddy list but he/she will still see you on their buddy list.