I'm not really sure how common it is, since I've never seen anyone do it, but why not just ban people from killing a T when they hit someone during an LR. You can kill them after they complete it or after they kill a CT but not during. I wasn't aware that this was a thing until ant8ters post about it and stunning saying it's allowed. Maybe it would steer people away from wanting a freeday tomorrow lr and, instead, do an actual LR like S4S, knife fight, etc?
Also since there's no wardens in this server, you can easily revoke that person from following X or X randomly giving Y a freeday. If they don't listen then shoot them. You're probably going to get called an asshole, but ¯\_(?)_/¯ who cares.