Hi Sarah,
My Mistake, The IsActive field that I mentioned when I originally made my last post is actually already a field listed in the Contacts Table. The reason you are unable to edit this field is because it is a Linked Field. Linked fields are fields which are pulling data directly from a different existing table and as such you will not be able to change its value.
The Idea behind my solution for you is still the same however. You'll need to add a YesNo field to your Contacts table with a name that does not already exist on the Contacts table (Perhaps call it ActiveContact)
Thanks Sarah. Let me know how you go!
Regards,
Ben