I allow early bag dropoffs and they seem to work fine. I make it clear that this is an extra I’m doing for them, and I cannot offer an early full check-in because I have to be fair to the previous guest just checking out etc.
I don’t know the layout of you place, but I wouldn’t allow them access to the room at all. Get the bags dropped off right by the external door, or even just outside if that’s possible.
I haven’t refused access to the toilet in such cases, it would sound mean. And even if there are toilets in the trains or planes, it’s not the same.
I do adminster the rest of the check-in stuff, like asking them about their schedules, giving them a map and such. That way, you don’t have to be there when they come in later.
This can work to the host’s advantage too - say they dropped their bags off at noon, 3 hours before std check-in. Most guests then come back later in the evening or the night, which can be a good thing for the host.
So I can say after having been burned by this, the damage from such scenarios can be mitigated.