Busy times should block off any and all availability, including multi-guest events.

Temporary work-around:

You can use this work around to block off multi-guest events for a specific event type. To do so, create holiday hours on the dates you want to block off (they take priority over any existing hours), and then set a time range which is too small for the event type's duration. For example, 12AM - 12:15AM.

See the attached image for an example.