Season of Worthy
With the release of Season of Worthy the old method of doing this no longer works. Bungie seems to have fully adopted the matchmaking offered by Valve's Steamworks infrastructure.
Some users have reported that blocking ports 27000-27100 but for the destination instead of locally seems to be working. While some report that it only works after they have restored the firewall defaults.
Why that is I have no clue, but I've added a button to the Settings tab so you can easily do this if you want to without having to use a PowerShell script.
Feel free to play around with the ports and report back any findings. You can use the Windows Resource Monitor to check what ports Destiny 2 are listening to.
Maybe try some ports from here and see if they work for you.
Changelog
[Added]: Button to restore local firewall defaults.
[Added]: Ability to open the Windows Resource Monitor.
[Added]: Button to reset settings in case something gets broken.
[Added]: Ability to add custom remote ports.
[Changed]: Ports have been updated to reflect some networking changes made by Bungie.
[Fixed]: Unable to remove custom rules if default rules were re-enabled.
[Fixed]: Some bugs causing text in the output console to get highlighted.