We found a few ways to bypass this. I didn't like the option above to log them out after a certain amount of missed chats so we found 2 ways to help ensure no one stays logged on when not around.

Option 1: first line of defense

In the idle timeout settings under chat settings, agents can make it so after a certain amount of minutes of inactivity zen will make them go offline. I feel like we still needed another line of defense. I just discovered this setting yesterday and one of my agents left before we could change his settings, and you guessed it he didn't log off chat. So I had a theory and after testing I was able to "log off" my online chat agent.

Option 2: removing and re-adding access

Let me start out that I have no clue if this impacts billing at all, I would imagine it doesn't and just allows you to restrict who has access to what. but I had that one agent who logged out before we could change his idle settings so to force him out I went to general zen settings-> Manage -> People and found my agents. then I toggle their Permission to chat off, which logged them out. I immediately toggled it back on to give Permission to chat again. This keeps them signed off until they log in next. Still unsure if there will be any repercussions of this, it just helped in a pinch.