I'd just open up the card and unplug the cable to the lights. It's not a bad idea to open up the card to reapply thermal paste/pads anyway if you're hitting the card hard, a lot of manufacturers don't do a great job with heatsink contact, thermal paste quality, or both. On the lower tier cards in their product stack half the time there won't even be thermal pads on the vrm or memory chips. And recently I saw a post where powercolor forgot to remove the tape from the thermal pads at the factory [0]. And no, in most countries they can't void your warranty for opening it up.
[0] https://www.reddit.com/r/Amd/comments/oyu1j6/thanks_powercol...