You can try sending a message the night before "Checkout is at 11am tomorrow - instructions are on the last page of the house manual in the lounge"...
I also have a note in my house rules that the space is theirs through the end of their paid stay & after that I reserved the right to host a naked twister party in the lounge. The threat of doughy middle-aged flesh seems to be enough to get them moving. (Really, I put the rule in to deal with squatters, but still applies to late checkouts)
So I've only had 2 guests overstay (w/o prior permission) - I just go in and start stripping beds & cleaning while chit-chatting about how fun it was to have them but that next guests are arriving in 3 hrs. They get the hint pretty quickly!