Capacity at overly congested airports is managed through the allocation of slots for landing and take-off according to the IATA Worldwide Slot Guidelines. Under this scheme, slots are allocated over a six month scheduling season, and requests are typically made in series i.e. requests are made for the same time and days of the week over a given period. Although allocating slots in series has the advantage of preserving schedule regularity, it causes blocking with a detrimental effect on capacity utilization. Blocking occurs when the allocation of a given series of slots prevents the allocation of another series due to capacity constraints even though the days of the two requests do no coincide. In this paper we investigate blocking mitigation strategies. We introduce a model and solution framework that minimises blocking while maintaining a desirable level of schedule regularity. We also investigate a strategy that reduces the effect of blocking by changing parametrically the length of the period defining a series a slots. We test both strategies under hierarchical and holistic slot allocation policies using real slot request data from a congested airport. We compare the two strategies in terms of their effect on slot scheduling efficiency and schedule regularity, and we provide slot allocation policy recommendations.