This is a big failing of AirBnB booking and pricing system. You can do this on Homeaway, Booking.com, etc., and in fact I struggle to think of any portal that has this issue.
Putting your nightly prices up distorts the pricing, as a) can make your property too expensive for those who do not want to stay for a month, b) could result in you breaching any Rate Parity agreements you may have with other portals, c) putting notes on longer stay discounts does not always work as your price and placement in search results is determined by your price entered in the system (manual notes about discounts is only visible if the guests clicks on your listing, which is highly unlikely if your listing is 30% more expensive than the average).
For example during the winter period, we give up to 30% discount for 28 nights or more. In order to exclude June-July-Aug-Sept (our peak periods) we will have to increase our nightly rates by 30%, which means we will be well out of the running for anyone wanting to stay for a week or even 2 weeks (average stay in this period is 2 weeks).
At the moment we remove the monthly discounts, the moment we put our prices for Summer period, which means we have to disable monthly discount at the start of each December, and activate it again between Spet-Dec.
I have to declare an interest as my husband's company develops and sells their own booking system, which we use for our own direct bookings on my website, which is also synced with AirBnB.
He cannot understand why AirBnB cannot fix this, as according to him, it is just a few lines of code. OK, most software engineers make such wild claims, but I happen to know he is right on this one. It took his engineers less than half a day to implement this function on their software, test it, and make it live, and that was over 5 years ago!!