Hello Warlocks!
It was not intended to allow someone to control another player that long. To address this problem, a change went in last night to decrease the duration of Subjugate Demon against players down from 5 minutes to 10 seconds (with diminishing returns following suit). There are some cool class interactions out there that feel niche that we hope to retain, within reason, especially in Player versus Player environments.
Thank you all for the bug reports, we appreciate it.