Deny access to one database, allow all the rest
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.