To block a port in UFW we use ufw deny command
ufw deny port/protocol
Examples
ufw deny 53
block specific IP address
The syntax is:
sudo ufw deny from {ip-address-here} to any
To block or deny all packets from 192.168.1.5, enter:
sudo ufw deny from 192.168.1.5 to any
To allow ssh service
ufw allow ssh
To see rules config file
cat /etc/ufw/before.rules|less
To enable firewall and active on boot
ufw enable