To block someone, first go to the Activity tab and tap on a payment or a request from that user. Next, tap the three dots in the top right corner and select Block user. You will not receive requests or payments sent from that person again unless you unblock them.
To unblock someone, tap on the Activity tab, and look for the notification informing you that you've blocked this user. It should look like Wero Update - You blocked a user. Tap it, and tap Unblock user. You will now be able to receive requests and payments from that user.