(1) Although purge should also stop any running instance, it may be possible it did not kill the running pfclient, and only removed its files. If this is the cause, then rebooting Pi will stop it.
(2) If reboot does not solve the issue, then may be the apt purge command somehow has failed. The pfclient is not unstalled from from repository by command apt install pfclient. Insted pfclient is downloaded by wget command, then installed by dpkg --install command.
Try using "dpkg --purge pfclient" instead of "apt purge pfclient".