There are plenty of hotels close to the conference venue. The closests one is Parkhotel Graz. Other, close to the city center include the Hotel Das Weizer, the Hotel Wiesler, and the Palais-Hotel Erzherzog Johann. For these hotels expect about 10-15 minutes walking to the conference venue. Outside the city center but within about 15 minutes walking distance there is the Star Inn Hotel Graz by Comfort.
More information about hotels and also a book page you find at the Graz Tourism web page: https://www.graztourismus.at/en/hotels-apartments.
Parkhotel Graz
Leonhardstraße 8
8010 Graz
http://www.parkhotel-graz.at/
Palais Hotel Erzherzog Johann
Sackstrasse 3-5
8010 Graz 8010
http://www.erzherzog-johann.com/
Hotel Wiesler
Grieskai 4-8
8020 Graz
http://www.hotelwiesler.com/
Hotel Das Weizer
Grieskai 12-16
8020 Graz
http://www.hotelweitzer.com/