ISPAC organizing committee is not handling the hotel reservations. Attendees should organize autonomously for the reservations.
ISPAC is just suggesting the closest hotels to the conference venue:
Starhotels Tourist: https://www.starhotels.com/it/i-nostri-hotel/tourist-milano/
Hotel Arcimboldi: https://www.hotelarcimboldi.it/
Beware: If any organization is approaching you with lodging support claiming to have connections with ISPAC, do not answer they are fake.
Please, also consider to make your reservation early. A major event is taking place in Milano starting on June 7th, the "Salone del Mobile" that will attract a huge number of attendees.