How can I punish employees who just go home right once their shift ends?

Change their contract to extend their shift upon request. Because if they’ve completed their contracted hours, then it becomes their time and there’s not much that you do. You can offer them extra money in the form of overtime or hours, but they don’t have to accept it. After all, they’re people with lives too. Ut let’s be clear; you’re only punishing people who just go home. Does this mean that you want people to hang out with out after their shift, maybe in a bar or an after-work social? Again, this isn’t part of the job unless it’s specifically within their contract. You might mock them for not joining in, but it has to be in good jest. What about employees who visit friends, go shopping, or meet up with family after work? They might even have another shift to start elsewhere. If you’re going to ask someone where they’re ging every time they finish a shift, it’s a little intrusive and quite frankly none of your business.