@Erin7082 you have two options to maximise revenues.
1) Change to a 7 night minimum over the high season(s)
2) Change to a 3 night min in high season and forcing only Friday/Monday checkins.
Option 2 may still get the odd blocked night but gives more flexibility to people and hence you may get more bookings. Conversely its a lot more cleaning! Option 2 also allows for 10 or 14 night bookings but I guess these are rare given how little vacation time folks in the States get.