Undiscolosed spotify p2p traffic was leaking across my VPN connection to my employer. Thanks spotify!
On mac os high sierra, you can block this with:
$ sudo vi /etc/pf.conf
Add the following line to the bottom of the file:
block drop quick on <interface> proto udp from any to any port = 57621
Where <interface> is the interface over which the traffic is leaking
Then, disable/enable the firewall
$ sudo pfctl -d && sudo pfctl -e
Kind of annoyed that a premium subscription lends you to something you can't readily opt-out of.