UFW

UFW (Uncomplicated Firewall) is a program to
manage a netfilter firewall.

Block specific IPs

To block the whole traffic from a specific IP to your computer run:
ufw deny from <ip> to any
This will not send a reject response, so the target of the block won't know he
was blocked.
If you don't care if the target knows you can also run:
sudo ufw reject from <ip> to any

List all rules

To show all rules you can run:
ufw status
or:
ufw status numbered
if you want to have a list of all rules with according numbers.

Remove rules

Locate the rule you want to remove by showing all rules (numbered).
Then run:
ufw delete <number of rule>