Just drop the hook and go for an Achilles Lock, or, even an overhand foot lock. There a lot of other possibilities there... in a real MMA mathch, you might want to stay with the finish on the hooked heel, but, as an above poster noted, when rolling with friends, don't be forcing leg locks. You can really hurt a friend, and that's just not the idea, there.
It's really, really, REALLY easy to hurt somebody, trying to foce a sub with a heel hook. I'd let it be and stand-up and roll again. Just that you got the position is enough to say you had the sub.