UFW Deny not really working
Solution 1:
Answering my own question after finding the answer: I found the problem was with ordering. Deny rules should go first, I had to show the rules numbered with:
sudo ufw status numbered
Then deleting the rule with:
sudo ufw delete 4
Then adding it first with:
sudo ufw insert 1 deny from 15.15.15.51
Then reloading with
sudo ufw reload
Now it's working.