Auditorium or theatre style room for rent - 70 seats?

Try:
"Full circle", Chaussée de Vleurgat, Ixelles, or just outside Brussels, "Le Rideau Rouge", Lasne.
Many Communes have such halls as well but you would have to call to enquire if they can be rented.

Chant d'Oiseau in Woluwe, a former monastery
https://www.chant-oiseau.be/

YTTAP, The Chant d'Oiseau venue can be rented also for meetings (we used to have annual assembly of co-owners there, without any catering).