TL;DR shoot them if they don't listen
Warn them that next time they are looting one of your kills even just to "check the loot" or "get looting exp" you are going to shoot them in the back of the head. And then make sure you follow through. If you ever find someone lie about what dogtag says then actually never play with them again.
You don't need to be aggressive about it. Between raids just say "hey we need to make it a rule that whoever gets the kill gets first pick at the loot. And they get to decide what they want to share."
But if you are in the raid like there was no discussion prior and they hop on the body you might be worried they will take some goodies. Say like "stop looting my kill or I'm gonna kill you, get up and cover me(/help me sweep more)"
If they still don't move, say it again all repetitive. Try to remember that everyone is scared of dying and sometimes lots of adrenaline. So try to sound assertive not aggressive
"stop looting my kill or I'm gonna kill you, get up and cover me right now or die" and then if they talk back be like "I'm gonna shoot I'm gonna shoot get up get up get up" lmao and then shoot them
If you are playing with randoms or new friends then you gotta kinda be assertive in raid, and if it's your actual friends or family just bring it up casually between the raids.
EVERY SINGLE DROP OF THE KILL BELONGS TO YOU. ONLY YOU DECIDE WHAT TO SHARE. IF THEY HELPED KILL IT DOESN'T MATTER THEY DIDNT GET THE JOB DONE.
Near the end of the raid ask everyone some of their best loot and offer to share or trade. In a safe area, somewhat before extract.
I'd you trust your guys to loot your kills that's cool, I just know how people can get greedy of it comes down to like a quest item they super need.
Also super separate tip but if your team doesn't have an obvious shot caller or leader, then you have slots more long term problems imo. It's good to have one guy to say when it's safe to loot and when it's not safe. Whoever has the most hours typically. Cause it gets problematic when people wanna loot your kill while you still feel the area is hot.