Column or Measure for NOT IN List

Thanks PaulDBrown!

The measure you provided definitely answered my question.  Now, I might not have provided enough detail as you mentioned.

This is the room table with data:

This is the room_booking table with data:

Next, from the report I'd like to show what rooms are booked and what rooms are available on a given date.  I'm thinking I need something similar to this:

From the above screenshot it seems to be working as I've described for my requirements, but I think the slicer is causing me problems.  It's using the check_out column fromt the room_booking table.  And the results in both tables (left table is booked, right table is available using the measure as a filter) are not consistent when I change the date in the slicer.

Any suggestions on how I can achieve this?

Many thanks!