Delete your monthly discount from 20% to 1% (there was a mistake in the past with 0% discount, I don't know if is fixed now, thats why I would reduce it to 1%) Host->Manage Listing->Calender and more->Pricing->Monthly discount
If you want to be strikt e.g. maximum 10 nights, then :
Reduce the maximum nights Host->Manage Listing->Calender and more->Calender->Availablity settings
I just did not offer a long term discount, I have had a few enquiries, in every case it would not have been a good fit and in every case they wanted a discount.