OP, you don't need to censor words here? You can even put cunt (not that it would make sense in this context).
I start winding things up with a few minutes to go anyway, but if it really isn't sinking in then physically getting up, putting on a robe and starting to tidy up usually gets them moving. If they want a shower and they're over already I tell them either no, or that they'll need to be no more than a couple of minutes (and if they have form for pissing about I'll ask them if they want a shower with ten minutes to go).
Saying you have another punter arriving soon is ideal - the idea of being seen seems to terrify them. You can also pull a face and tell them that you desperately need a poo.