Question about Sql Accounts and Deletion Management

Question about Sql users.  I have a sql user that is added in the security section of the actual database.  I have the same user located in the instance level of security.  I deleted the user at the instance level but the user remained at the database level.  What is the proper way to delete a user if I totally want to rid the instance and all databases of this user login.

Also, all of the users in question should not be a owner of the database.

Question.... is there any danger in data being inaccessible after deletion of these users.

also, if i happen to delete a database owner..... i can always reassign that role to another user from an SA login.... Please verify.