Come on now... Really? You do that, you'd be unemployed in a second.

I have done that. A customer spat on another employee and I kicked them out right then and there.

But it's not an annoyance with having customers. It's an annoyance of them not adhering to when your store closes.

If the company policy is to let customers continue shopping, then they are allowed to continue shopping. If the company policy is to kick them out at closing, then you kick them out at closing, or 15 minutes after, or whatever. Many stores make a P.A. announcement "We're closing in 15 minutes and will be asking you to leave in 25 minutes".

It's really simple. Have a clear policy.... and abide by it.

What's not acceptable is getting mad at customers for following whatever policy the company has, or for expecting them to know it if you don't tell them.

You seem to think it's black and white, and it really isn't.

It really is. Be kind, be courteous, but enforce the rules. If the store is closing, then as kindly as you can, tell the customer to leave. If the store policy is to allow them to stay, then make sure you get paid for your work past closing. If there's any confusion or ambiguity, get clarification about what you should do. If your company gives you an impossible or contradictory task, report them to the department of labor. If you fear losing your job and you need that job and don't want to report it because you know it will get back to you, then job hunt in your off time or accept that your day will sometimes be a little longer than you think it ought to be.

If you don't do that, then you are going to be perpetually disappointed because you have unreasonable expectations. You want the world to work one way, but know it works another, and don't do anything to change it. If you get upset about that set of circumstances, then you're just always going to be upset.