I think the simplest thing would be to reduce the allotted idle time in matchmade strikes and crucible matches. It's pretty long right now, but if they dropped it to, say, 3 minutes, it would just get them kicked. Everything else like patrol, missions, raid, etc can stay the same so people can go get something to eat or use the bathroom without fear if getting kicked from solo stuff or from your raid. If you need to be gone longer than that you shouldn't have joined matchmaking, and idlers would be replaced by actual players more quickly.