As someone who's had a friend do exactly what you did, the only good advice is to get a lockring tool and adjust the way you ride your bike. Since you're specifically talking about being brakeless, I'll try to give brakeless advice. Haha. ^^;;

1. If you're going at a full sprint, don't try to stop abruptly with locking your legs like that. Your weight is probably mostly on the backwheel, and while that helps you stop faster, it's just a bit too much force (imo) for your cog/lockring to handle. You can do a more leaned forward skid (not excessively so) while trying to slow down your cadence normally to slow down. Basically saying that you need to slow down gradually rather than abruptly.
2. Learn to ted shred. I mean, it's not hard. You just jam your foot between the wheel and seatstay. But it slows you down fastest in my experience with the least amount of effort. You're paying with your shoes to do this though.

Well, I guess that's it. Both of them require you to think ahead and know what's happening around you. Abrupt stops aren't really ideal or suggested on a brakeless fixed gear. In some cases, it might be better to avoid rather than stop. But it's best if you've just thought far enough ahead that you know what's happening and can slow down as necessary.

I don't know if I helped, but good luck.