To block a contact in imo, follow these steps:

1. Open the imo app on your device.

2. Find the contact you want to block in your chat list.

3. To block a stranger that you don't add as contact, 

please open the chat sent by unknown contacts > click "BLOCK" on the top.

To block a friend in your contact list, 

please tap friend's profile picture > tap his profile picture again to enter his profile page > Block.

4. Once you block a contact, they will no longer be able to send you messages or make calls through imo. 

Additionally, they cannot invite you to groups and voice rooms.

5. If you change your mind and want to unblock the contact, 

you can do so by going to the imo settings.

6. In the "Security & Privacy" settings, look for the "Blocked List" option and tap on it.

7. You will see a list of all the contacts you have blocked. 

Find the contact you want to unblock and tap on their name.

8. A menu will appear with the option to "Unblock" the contact. Tap on it to remove the block.