To unblock a certain user, go to your admin dashboard and find the menu on your left. Click "Limit Attempts" -->“Blocked” and hover your mouse over the necessary address in the “IP Address” or "Email" column. Click the "Reset block" caption and the IP or email address will be unblocked. Use “Bulk Actions” to unblock a group of users: mark the addresses that have to be unblocked, choose the “Reset block” action and click “Apply”.
Click the "Add to Whitelist" caption and the IP or email address will be whitelisted.