I was not able to delete a user in my SQL Server. The error message that was thrown was something like: "Can not alter user dbo". The user had previously had a membership connection to a database.
The solution was to temporarily detach the previous database in order to delete the user from SQL server.
- Right click the database
- Tasks
- Detach
- Check the "Drop connections" box
- Click Ok
- Right click the user under Security -> Logins for SQL Server
- Delete the user
After that I could restore the database and re-add the user to it.
In order to reattach the database:
- Right click "Databases" under SQL Server
- Attach
- Add
- Select the database you had detached
- Click ok