You can actually have any message you don’t want cleaned from the chat room. Nevertheless, this will only work on the side of the user where messages you have deleted will only be deleted on your device as it will show on the device of your friend.
If you still need to delete the messages, go into the chat room and locate messages you want to wipe off, click and hold on the messages and select DELETE or click on Navicon in the alternative and select EDIT MESSAGES and then select the messages you want to delete and click on bin icon at the bottom right.
Extra Tip: Block and Unblock Mobile Phone Contact Numbers on Viber
You can always add those people you want to avoid having contact with to the block list. Once you have done that, those in your block list will not have the opportunity of having a direct conversation with you or call you with Viber.