Deny access to one database, allow all the rest How can I deny access to specific databases from a login?  I know that I can walk through and make the login a db_owner on all the ones I want the login to have access to, but how do I do the opposite?  I want the login to have access to everything but the ones that I specify
I would highly suggest you not assign a login to db_owner just "to have access to" a database unless you want them to have "admin" privileges in that database.

You also have to consider what permissions the login has at the instance level first. Assigning a user to db_denyXXX will have no affect if the login is sysadmin or securityadmin privileges.

If we are talking about a "regular" login on the instance you prevent them from accessing the database by not mapping that login to a user in the database. If no database user exist in the database for the login then they cannot connect to it, as long as they do not have higher privileges at the instance level.