Note: This is known to work for a machine acting as masquerading firewall for an entire network. 
I wanted to block connections to certain outbound ports. After some trial and errors I found something that works
Add the following to /etc/ufw/before.rules:
-A ufw-before-forward -s <SOURCE IP> ! -d <LOCAL NET>/24 -p tcp -m tcp --dport <DEST PORT> -j DROP
it’s as simple as that.
Remember to reload all the rules:
$ sudo ufw reload