MyVR allows inquiries for less than the minimum nights identified in settings. That way the inquiry is still received and allows the chance to sell the person on a longer stay if desired.
If a strict minimum night policy is desired, the best route is to set up an auto-responder rule for inquiries which do not meet minimum requirements. This would automatically mark the inquiry as declined. That way, an inquiry is still received, the prospective renter's information gathered, and the ability to nudge the customer towards a longer stay is retained, if desired, in the automated response they receive.