Since the landlord is legally prohibited from controlling the tenants behavior, how do you justify holding them liable?^ Solutions depend on where the party was held. On campus, then it's the university's responsibility (ie. plans should have already been in place) on how to deal with the nonsense regardless of Covid's presence.
On private rental property, then it's the party's organizer + landlord. Police + bylaw charges on both parties should straighten them out.
The housing situation keeps getting worse, and part of it is people like you wanting landlords to keep taking responsibility.