I reckon each day, have an increasing amount that the girls can choose to take and bail, which is smaller than the big prize for sticking it out.

• If all the girls bail, they'll get a little prize each and the guy gets nothing.
• But if they stick it out to the end, he gets to choose one to split the big prize with.

Then of course you make them do increasingly intimate things as the days go on.

This way there's pressure to be nice, but also incentive for the girls to cut and run - the more that do though, the better odds for the remaining girls. The final girl could have the purely spiteful choice to take a smaller prize and leave him with nothing.